Do women experience more bonding hormones than men after sex?
Does Oxytocin increase more so for women than for men after sex?


Answer: Yes, Oxytocin level shoot up for female after sexual. It is highest at orgasm, which is the main cause women could have multiple orgasm.


  Someone told me that it increases 4-fold for men and 12-fold for women
  (unconfirmed).


Answer: The level of oxytocin actually increase 5 times during sexual intercourse. Though some online article claim the male and female level as much you said, but it is not confirmed in humans. 


  Is there any observable or scientific relationship between sex, Oxytocin and the desire for a monogomous relationship with a sexual partner?


Answer:

Take a look at this experiment done on people to test Bonding:


  Bonding is an important part of human romantic relationships, and
  research has suggested OT as a possible mediator of this effect. A
  2005 study by $Grewen\ et\ al^{4}$. found that couples who reported a greater
  amount of partner support showed higher baseline OT plasma levels.
  These couples also showed higher levels at 4, 7, and 10 min following
  warm partner contact. For women, OT levels increased sharply 7 min
  following warm partner contact, regardless of partner support, while
  male levels remained unchanged. This study demonstrates a
  relationship between supportive romantic bonding and OT, and shows
  that warm contact has a greater physiological effect on women than on
  men.[1]


A very famous study done on rats confirm that Vassopressin and Oxytocin is actually the reason behind monogamous relationship.


  Almost a decade ago, Thomas Insel, Director of the Center for
  Behavioral Neuroscience at Emory University, and colleagues at the
  National Institute of Mental Health and the University of Maryland
  implicated vasopressin and oxytocin in controlling the preference for
  particular partners in both male and female prairie voles. These
  investigators found that giving a male vasopressin causes it to stay
  with its mate, whereas blocking this hormone prevents a pair-bond from
  forming. The scientists saw similar effects in female prairie voles,
  with oxytocin determining the extent of pair-bonding. In contrast, the
  administration of these substances had no influence on social
  interactions in the promiscuous montane voles. Surprisingly,
  vasopressin and oxytocin are naturally found at similar levels in both
  prairie and montane voles.[5]




Source:

[1]. The role of oxytocin in mating and pregnancy

[2]. http://en.wikibooks.org/wiki/Relationships/Hormones#Oxytocin

[3]. The role of oxytocin in relation to female sexual arousal.

[4] Grewen, K.M., Girdler, S.S., Amico, J., Light, K.C., 2005. [Effects of partner support on resting oxytocin, cortisol, norepinephrine, and blood pressure before and after warm partner contact](Effects of partner support on resting oxytocin, cortisol, norepinephrine, and blood pressure before and after warm partner contact.). Psychosom. Med. 67, 531–538.

[5] http://www.smart-publications.com/articles/oxytocin-the-real-love-hormone