Not to get too off topic, but it's been one of my causes for re-entering the film industry. After the corporations bought out the major studios in the 70s you still had a plethora of G-rated and PG-rated films that all ages could enjoy, and they weren't strictly family fare either. They were adult films about relationships, politics, personal issues without any swearing or sex. Some of them were boring as hell, but others, dramas and comedies, were excellent or at the very least, quite good. That was in addition to all of the R-rated stuff being produced.
Now G-rated or PG-rated films are strictly meant for kids, and that mainstream adult audience that used to get packed into movie houses (professionals between 25 and 50) are now all but vanished because of the quality of films and the film going experience.
On alien sex; I think I've heard mention that Marc Miller wanted to get Traveller away from sex and violence in his game. Me, using sex (even inter-species relationships) seems to be reaching for something to spice up an otherwise mundane story; i.e. add sex because we've explored everything else.
If sex is your thing, then go for it. Me, I prefer to write stories and adventures that deal with adventuring.
As far as alien mating habits go...hell, I don't know. Do the Virushi do somekind of ceremonial dance before the honeymoon? Are Vargr like their ancestral progenitors back on Earth and just have at it out in the street when the "urge arises"? Do the K'Kree massacre a bunch of meat eaters before nuptials? Do Hivers...heck, I can't even imagine what they do. Bwaps, Vegans, some of those crazy whacked out races from TNE/T4?
I don't know, and really don't care much.
I can see an adventure where the players have to stop a wedding, and there this topic might come into play. Beyond that, unless you have some evil scientists type splicing DNA together to make monstor-soldiers or something crazy like that, I don't see this as being very pertinent to sci-fi in general.
However, if you do like this topic, then one anime film comes to mind; "Dirty Pair; Affair on Nolandia". It's got some of the issues in this thread in that film. Be warned, it's an R-rated viewing experience. The Gall Force anime series deals with
There's a couple of other films out there.
Okay, now I'm really done with this topic.