> only questioned why he would have brought kids into the "union"
For a lot of people, building a family is a duty you embrace with your household partner. It's why you exist in the first place. It's why you get married and share a home with somebody at all.
Perhaps, if you're lucky, your children are a fruit of love, or perhaps, if you're horny, they're a fruit of passion.
But for a lot for such people, having and raising kids is the entirety of why you get married, and is the rationale for you might not marry for love or passion in the first place.
Marrying the person you're most attracted to or have the most fun with or whose pants you're most eager to get into is a very culturally specific practice and frankly, even where it is an aspiration, its one that a lot of people just don't luck into. But they nonetheless feel an obligation, and even desire, to form and raise a family anyway, and so they march ahead and get it done, hopefully with somebody that they respect as a partner and who reciprocates the same.