And in fact female-female marriages (though at least in theory not sexual) have a long history in some cultures as a means for infertile women to gain heirs...

It'd be inaccurate to call those gay marriages as some of the cultures they are practised in at the same time have been very oppressive when it comes to homosexuality (though presumably such marriages have been used as a means to hide lesbian relationships), but the fact remains women have been able to marry women with the explicit intent of having shared children for hundreds of years... [1]. And they're not few:

" Kevane (2004) estimates that approximately 5–10 percent of the women in Africa are involved in woman-to-woman marriages. "

[1] http://www.osisa.org/buwa/regional/female-husbands-without-m...

Exactly right - this will count as a tragedy and there is lots of research aiming to help such people to have kids.