How do monks transcend the desire for relationships & sex?
Is it with metta towards the opposite sex or just seeing the unattractiveness of it that craving reduces?

I think it's both of those -- with "unattractive" meaning:


Something like graveyard meditation
Seeing it as dissatisfactory (dukkha) or "not permanently satisfying"
Seeing consequences


I get the impression from watching some of Ven. Yuttadhammo's videos that it's also a matter of developing some dispassion towards "feeling".

And I think perhaps it's a matter of discipline (e.g. the vinaya) as an alternative to pursuing sensuality.

Another example of that, for a monk, is feeling hungry and yet not eating (except properly).

I suppose it's also a matter of habit, changing or attenuating existing habits (if any), and avoiding causes or occasions (e.g. being alone with a woman).