There are a couple biological factors that could rain on our parade here.
Mechanical isolation: Maybe our fun-time organs just don’t fit together and there’s no way to put Tab A into Slot B.
Behavioral isolation: Maybe their ways of signaling sexiness are about as arousing to humans as graphic descriptions of late-stage syphilis.
Habitat isolation: Maybe the aliens can only get in the mood if they’re immersed in sulfur or in Marianas Trench-level ocean pressure.
Gametic isolation: Even if the previous things are dealbreakers for most people, there’s still a contingent of people who really want to have alien babies. [Source: Deviantart.]
But different Earth species being able to have offspring at all is really rare, and you need very similar species; donkeys and horses, tigers and lions, slightly different varieties of coral. Mules and ligers don’t count as species, since they’re sterile or have devastating genetic failures within a generation or two.
So, here are all the things that need to happen perfectly if we’re going to have alien families:
1. We find aliens.
2. Those aliens understand how human sex works.
3. Those aliens are sentient enough to give consent.
4. Those aliens need to have genitals perfectly compatible with humans’.
5. Those aliens must be able to survive and be capable of arousal in comfortably Earth-like conditions.
6. Those aliens are sexually appealing to humans. [This is the easiest hurdle to clear by many orders of magnitude.]
7. Those aliens must have chromosomal setups pretty much identical to humans’.
8. Those aliens must be culturally similar enough to humans that forming lasting relationships and families is remotely possible.
I don’t blame erotica writers for hand-waving it all as “they’re conventionally-attractive humans but with some weird horn setup or something, shut up, it’s porn, if you want ironclad biological accuracy go read National Geographic or whatever.” But just once, I’d like to see an interesting exploration of the idea from a hard-science point of view.