I don't really care about the moral, just that it doesn't make any sense. I mean, I could live with it, if in fact my sims would actually be romantically involved with someone while I'm not looking, and then became pregnant and THEN actually had the baby. But, it annoys the heck out of me that they just suddenly appear from nowhere! As adults! I like to keep my game quite realistic, so having these random (adult!! I just can't get past that...) children pop up and not being able to do anything about it really annoys me.
I searced around a bit, and it seems the WA version of Awesomemod might fix this. At least it says in the desctription: "13. Active/Sacred Sims will no longer suddenly find themselves the parent of some random vacation locals." I'm hoping for the best. I haven't had the chance to try it out yet, but once I get the mod installed, I'll see what'll happen. Too bad it won't have any effect on the existing children. :/
Oh yeah, and I think someone mentioned here or in some other thread about this same issue, that their sim had the "heartbroken" moodlet after the apparent other parent died, as if they saw it? That happened to me too.