Is Maslow's hierarchy of needs really accurate at labeling sex as a physiological "need"? Is Maslow's hierarchy of needs really accurate at labeling sex as a physiological "need"? If so, would that mean that single/celibate people cannot live self-actualized lives unless they have some sort of sexual outlet (e.g. masturbation)?
This is a trickier multi-part question to adjudicate than you might think.

Is Sex A "Need" (Physiological or Otherwise)?

Definitions of "Needs", "Motives", etc., are dime-a-dozen. Though I don't necessarily agree with all the ingredients, I like how well explicated the criteria by Baumeister and Leary (1995) are, according to whom a fundamental need should: 


produce effects readily under all but adverse conditions
have affective consequences
direct cognitive processing
lead to ill effects (such as on health or adjustment) when thwarted
elicit goal-oriented behavior designed to satisfy it (subject to motivational patterns such as object substitutability and satiation)
be universal in the sense of applying to all people
not be derivative of other motives
affect a broad variety of behaviors
have implications that go beyond immediate psychological functioning


To me, sex meets several of these criteria (e.g., it clearly directs affect and cognition, is universal-ish [asexuality notwithinstanding, a point a return to in a moment], but is questionable for a couple that, in my opinion, are crucial for bona fide "need"-status. 

First, it is not clear that thwarting of sexual desire leads to ill effects. Here, Baumeister and Leary (1995) are referring to big-ticket consequences, not affective tiddlywinks. In their words: 


  A motivation can be considered to be fundamental only if health, adjustment, or well-being requires that it be satisfied. Also, motivations can be sorted into wants and needs, the difference being in the scope of ill effects that follow from nonsatisfaction: Unsatisfied needs should lead to pathology (medical, psychological, or behavioral), unlike unsatisfied wants. (p. 498) 


And while it is true that sexual frequency is positively associated with well-being, the benefits eventually "level out" (Muise et al., 2016), and there is not a clear link between a "lack" of sex and more serious medical or psychological pathology. And when one considers the occasional medical/mental health costs of engaging in sex (e.g., Meier, 2007), or the considerable efforts people go to to sometimes conceal their sexual activity (e.g., Olson &amp; García-Moreno, 2007), it's not at all clear that engaging in sex is always a plus for mental or physical well-being. 

It's also debatable whether the "need" for sex is it's own drive, or rather, derivative of other bona fide need-status motives. Take Meston and Buss's (2007) work on identifying reasons (over 250!) people give for engaging in sex. And when you look at their list of Top 50 endorsed reasons (many of which are similar for men and women), many of them seems to be about other drives/motives that may be more fundamental (e.g., social, relational, growth-oriented, etc.,). This issue spills over into the "ill effects" discussion as well; even in situations where a lack of sex co-occurs with more serious pathology, it's difficult to decisively isolate the pathology as a byproduct of the lack of sex, specifically, as opposed to deficits in other fundamental needs (e.g., belonging/security). 

Tl;dr: sex is probably better thought of as a strong, widely-shared "want", rather than a bona fide "need"

Need-Status Notwithstanding, Is Sexual Desire Primarily Physiological?

Sexual desire is surely physiologically driven to some non-zero extent. The association between physiological indicators (e.g., hormones like testosterone) and sexuality, however, is often a lot more complicated than the neat linear stories we like to tell (e.g., "Men have stronger sexual desire because they have more testosterone!"). An exhaustive review of this complexity would be overwhelming, but the work by Sari van Anders (e.g., her 2012 review + study) would be a good place to start. There's also plenty of evidence for non-physiological contributors to feelings of desire, such as those that are relational (for reviews, see Mikulincer &amp; Shaver, 2007,  Muise et al., 2018, to name a few).

Tl;dr: Sexual desire (need or not) is determined by both physiological and non-physiological factors alike 

Need- and Physiological-Status Notwithstanding, Is Sex Needed for Self-Actualization?

Probably not, but the state of the world makes it very difficult to cleanly appraise this. Looking at self-actualization in asexually-idenfitying folks would initially seem the clearest route to testing this claim, and if you took note of some of the mental health challenged faced by that community (e.g., Yule et al., 2013), you might be tempted to conclude that there was evidence that sex is needed for self-actualization. The problem, however, is that there is pretty clear evidence that asexually-identifying individuals are targets of prejudice and discrimination (e.g., MacInnis &amp; Hodson, 2002), and so just as--if not more--likely that their mental health challenges are the result of chronic stigmatization of their identities/sexual orientation (which is currently the way that experts like Yule and Brotto and their colleagues conceptualize asexuality vs. alternatives conceptualizations [e.g., a sexual dysfunction], based on their research, see Brotto et al., 2014; Brotto &amp; Yule, 2017)

tl;dr: Evidence currently challenges the construal of asexuality as a disorder, and mental health problems in asexual communities cannot be divorced from the broader social context in which these communities are routinely stigmatized--all of which is indirect evidence that it is premature (if not entirely mistaken) to argue that sex is needed for self-actualization

References

Baumeister, R. F., &amp; Leary, M. R. (1995). The need to belong: desire for interpersonal attachments as a fundamental human motivation. Psychological Bulletin, 117(3), 497-529.

Brotto, L. A., &amp; Yule, M. (2017). Asexuality: Sexual orientation, paraphilia, sexual dysfunction, or none of the above?. Archives of Sexual Behavior, 46(3), 619-627.

Brotto, L. A., Yule, M. A., &amp; Gorzalka, B. B. (2015). Asexuality: An extreme variant of sexual desire disorder?. The Journal of Sexual Medicine, 12(3), 646-660.

MacInnis, C. C., &amp; Hodson, G. (2012). Intergroup bias toward “Group X”: Evidence of prejudice, dehumanization, avoidance, and discrimination against asexuals. Group Processes &amp; Intergroup Relations, 15(6), 725-743.

Meier, A. M. (2007). Adolescent first sex and subsequent mental health. American Journal of Sociology, 112(6), 1811-1847.

Meston, C. M., &amp; Buss, D. M. (2007). Why humans have sex. Archives of Sexual Behavior, 36(4), 477-507.

Mikulincer, M., &amp; Shaver, P. R. (2007). Attachment in adulthood: Structure, dynamics, and change. New York, NY: Guilford Press.

Muise, A., Maxwell, J. A., &amp; Impett, E. A. (2018). What theories and methods from relationship research can contribute to sex research. The Journal of Sex Research, 55(4-5), 540-562.

Muise, A., Schimmack, U., &amp; Impett, E. A. (2016). Sexual frequency predicts greater well-being, but more is not always better. Social Psychological and Personality Science, 7(4), 295-302.

Olson, R. M., &amp; García-Moreno, C. (2017). Virginity testing: a systematic review. Reproductive Health, 14(1), 61.

van Anders, S. M. (2012). Testosterone and sexual desire in healthy women and men. Archives of Sexual Behavior, 41(6), 1471-1484.

Yule, M. A., Brotto, L. A., &amp; Gorzalka, B. B. (2013). Mental health and interpersonal functioning in self-identified asexual men and women. Psychology &amp; Sexuality, 4(2), 136-151.