Oh please. Saying you'd never go to the wedding, as DH has done, is homophobic.
Parents are welcome to feed sad about maybe not becoming a grandparent in the usual way or etc, but those thoughts should remain in their heads. Gay people are no more at risk than anyone else if they practice safe sex. Also, it truly doesn't matter if someone identifies as a lesbian then changes their mind. Young people are entitled to explore their sexuality. If you have 'concerns' about someone's sexuality go and sort them out in therapy. It would never get phrased that way about a straight couple, when in reality I'd be far more terrified of my daughter dating men one day due to the sheer amount of women who experience domestic violence from men.
Also the amount of people surprised by the prospect two teens may want to have sex...this has been going on for as long as the human race has been here. Yes you don't encourage it, but if teens really want sex, they will, and all you can do is encourage safety. The age of consent is 16, but the law is very clear that that doesn't exist to punish two fifteen year olds.
Also, commenting on the mental health of lesbians and comparing someone exploring their sexuality to a rebellion is homophobic. Learn some history. It wasn't too long ago homosexuality was classified as mental illness.
Anyway, OP, you need to back your daughter and not tolerate your husband's views. I'm a lesbian and my dad called me vile and abnormal when I came out, and it's stuck with me. Gay people already experience disproportionately more mental illness, don't let your daughter be one of them.