New post since this is unrelated to the previous one:
I've allowed a number of mercenaries to stay at my fortress, and the wife of one of them visited, left, returned, and asked to stay (she's also a merc). I arranged for food in their inn room (yes, they share, and did so nominally even when she was just visiting):
Lesson 1: Mercs don't care about burrows even if their squad is inactive, so getting them both in the room at the same time required some fiddling.
Lesson 2: She won't get pregnant, because mercs do not go down to an "no job" idle state, but rather a "mercenary" one that prevents any useful activities to be performed.
Conclusion: Unless you want some early meat shields and a very modest amount of equipment mercenaries are currently vastly inferior to various kinds of performers (I assume scholars work as well, but haven't gotten enough experience with those), since the latter can ask for citizenship, and when they do they're fully under your control, and thus can take up military positions as well. None of my performer citizens have had their spouses visiting, and I haven't tried to persuade any of the unmarried ones to marry (using the pre honeymoon suite rather than DFHack, if so).
As an aside, I've had resident (but not citizen) performers harvest and trade (the trader was locked up in the pre honeymoon suite at the time), and I've even had a non citizen gremlin trade (the time was running out and the trader was hauling a heavy object a long distance). Mercenaries, on the other hand, only satisfy their needs when not engaged in military stuff (and does that just as poorly as citizens do, i.e. ignore order to continue to perform "interruptible" praying, socializing, and reading instead).
Finally, to relate to the previous post anyway, the female merc won't get pregnant with either of the male ones as explained above. It may still be possible to get them to marry, though, as I don't see anything that would block that from happening, but that probably won't achieve much, while requiring a fair bit of effort.