Are you kidding me? That's disgusting, immoral, unnatural and grotesque.
To think that.. my god that's just too disgusting to even imagine. I mean come on....
Having sexual relations with humans, such an idea is an insult to even imagine for a bold and mighty Sangheili warrior like myself. To sleep with a human woman is to pour acid upon my reproductive organs.
Okay but seriously:
I'd be open to the concept but of course it does depend on how said aliens reproduce. I mean if they eat each others feet in order to have sex, that could prove very uncomfortable to say the least, plus if they looked nothing like human beings it could prove to be a turn off, probably on both sides.
I mean it could be classed as bestiality, on both sides too. Because you aren't just going for a separate subset as your own species, but rather a whole other species.
But still, love is love I guess. In the end if I liked their personality and got on well with them and had fun with them, then it wouldn't matter. True in Mass Effect I passed on any coitus with Liara (though I went after the Consort) but that was because personality wise I preferred the tough Ashley to the sensitive Liara.
(I swear I'll stop mentioning Ashley when people stop ragging on her in forums)
So yeah, what I'm saying is biologically it all depends on how the two species interact biologically, socially and if they can appeal to each other on an emotional level. If all that can work perfectly then I see no reason why not.... except for that I just released a radioactive agent into the air that is sure to rot out the wombs of ever human woman in a thousand mile radius.
MWA HA HA HA HA HA!!!