I haven't had sex, and I'm about as far from desperate as you can get without being in an alternate universe where conscious thought is a physical impossibility. However, just for you OP, I'll give you my answer if I did have sex; "Is it something my potential future partner wants?" Not just an impulse thing either, is it something they really, really, really want to do at least once in their life-time (if it was I'd do it for them, and the 'really' want thing isn't that I'm opposed to it, far from it, I believe consentual polygamous relationships should be promoted as long-term things, rather than just the social norm of "One night threesomes", it's that the partner I'd want to be with wouldn't be someone who jsut wants to try a threesome for shiggles, but who views it as an experience that is worth trying (you know, one of those 'even if it isn't for you, you should try it at least once' things).
Man, that ended up being a shit-tonne longer than I thought it would be, I apologise to any who finished that and just went "Wow, what a waste of *however long it took to read*".