Oh, I can so much relate! I (mis)spent my teenage years failing to convince my dad I might be gay, and when I finally met DP (after a rather circuitous route ...) aged 31, he was genuinely shocked and surprised because 'I naturally assumed it was a phase'.
DP getting pregnant really sent him into a spin, because he could relate to her, just about, when he convinced himself she was more or less a strange kind of bloke who happened to look female. But the pregnancy really was a challenge.
MIL is more like your mum in focussing on the fact we're not in an opposite-sex marriage (in fact we're not married at all). She can't get her head around the idea that I'm 'mum' too.
It's a strange and slightly lonely thing trying to have a baby in a lesbian relationship, but this thread is the best place to be while you're doing it!