…how so?
Mystique’s body clearly contains some organ or other element (maybe on a celluar level) that non-shapeshifters lack. The body of – oh, let’s say Natasha Romanov, another hot redheaded superspy, only lacking shapeshifting powers – does not. If Mystique were to impersonate the Widow, she would obviously have to retain her own unusual shapeshifting factor to change back (or into, say, Siryn, or Mary Jane Watson, or Jean Grey), and thus has not completely changed.
Also, she doesn’t can’t imitate non-muscular abilities. If she emulates Jean Grey, she doesn’t become nigh-omnipotent; otherwise, why would she ever change back?
She doesn’t get the special abilities or those she shape shifts into… but base physiological functions seem something her body can handle. It may be inconsistent, but considering she has shapeshifted into a multi-armed version of herself with two functioning faces…sperm production seems pretty simple.
It’s all just X’s and Y’s anyway, right?
Oh. I agree that Mystique could become functionally male, and I much prefer to think of Nightcrawler as being helr/his son by destiny than as her son by a devil. (Chuck Austen can go fuck himself, which Mystique, incidentally, could also do easily). I was just nitpicking the word “completely.” 
They’re also both from Africa, so that’s something anyway? I dunno, I got nothin’.
Shrug, one of his powers appears to be “makes any telepath wets his/her pants”. Betsy is the exception, unless they hooked up at some point when I wasn’t looking.
Oh, that’s actually explained. Maybe I’ve been wooshed, but their tribe’s first ancestress made whoopie with a wolf; both the elves and their wolves are descended from that pairing. Every wolfrider (humanoid or canid) is blood family.
She might be able to produce sperm, when she assumes male form, but that doesn’t necessarily mean that the sperm would be viable.
Howard the Duck and Beverly Switzer - Howard is a duck, Beverly…is not.
It is if the writer says it is, until a later writer says it’s not.
Would it help to think of Howard as just a person from a place where people happen to be shaped like ducks?
I thought not.