I think we're all overlooking a key problem... I refer to an episode of Futurama entitled "The Deep South" (didn't see any clips of the scene i'm talking about and i ain't posting a full episode... you can look it up if you want to see.) And I quote, "oh Fry! I want you to make a mer-woman out of mer-me!"... a few seconds later... "wait, i'm confused... how am i supposed to... i mean... with the tail and all...?", "oh no! i'm not your first am i? i lay my eggs and then i leave and you release your fertilizer"... a few seconds later... (fry, fully dressed comes racing out of the house) "why couldn't she be the other kind of mermaid?! with the fish part on top, and the lady part on the bottom!!"
I think that answers the question.
...also all those bits about Space AIDS and all the middle school hygiene class things... but from space. still scary stuff.
In short, there are 4 critical details in this alien sex thing...
1) we are physically compatible. (as stated above)
2) she is free of any disease or parasite, and the act and aftermath will have no detrimental effects on either of us (i.e. she doesn't try to eat me afterward, our various fluids aren't toxic or corrosive to each other, etc.)
3) she is reasonably attractive. (no Predator aliens, for example...)
4) she is a willing participant. no matter what the fantasy, rape is still rape.
Not knowing any of those leads me to say "no alien sex".