this OP is very troubling to me.. I think that restrictions like this would so limit the possibility of finding one’s beshert. IMO it SO shouldn’t matter if the boy/girl is an only child or one of 17. consider the fact that if one were to marry a BT that then they wouldn’t even have that other set of parents to go to for Shabbosim & Yom Tovim. I would hope that one’s spouse is chosen on the basis of lev tov & yiras shomayim, not on family size. it just seems so shallow to me. jmho…
(I have an only child son-in-law, & a daughter-in-law who’s 1 of 9 ka’h).