Hasnt anyone considered the very obvious solution to a married couple producing offspring with others without having to betray their vows by having sex with another i.e. the star trek equivelant of a turkey baster.
If Bashir can shift the baby from Keiko's womb to Kira's on a transport shuttle with limited equipment he's clever enough to be able to figure a way to use the defiant medical bay tech to artificially inseminate people. This would have been a good solution for any crew members who didnt find anyone they particularly wanted to hook up with but still wanted offspring, or any same sex couples that might have occurred. For a couple whose species were unable to interbreed without the assistance of much more advanced tech than was available on the planet it makes sense to find willing crew mates to help with artificial insemination.
Also since the early days of the colony were busy surviving and it wasnt that large a crew i could easily see child raising duty being shared by all adults not just the childs biological parents. The biological parents would still have more claim/influence but the child would have a very large number of aunts and uncles constantly looking out for them. Children raised just as much by the community as by their parents which would also make it easier in cases where a child was the result of artificial insemination for a married couple unable to breed for all the parents to be equally as involved in the childs upbringing.