My best friend is gay. As are several more.
I have a transgender child (well, nearly adult now).
When children were younger, gay and lesbian aren't words that we used, just partner, love etc.
I don't think your DH is homophobic, just unclear on how to address relationships with children. The easiest way is to avoid categorizing them. A m/f marriage isn't a "straight" marriage, so why should a "gay" marriage be isolated as something that is "other"? Boyfriends/husbands/girlfriends/wives are boyfriends/husbands/girlfriends/wives regardless of the partner's gender.
I can only speculate, of course, but perhaps a gentle chat after your DCs are in bed might throw a bit more light on this.
Funny story about gender, actually, My youngest child has physical and intellectual disabilities. She wears leg braces and always gets to choose whatever shoes she likes to go with them. Most recently, she found a pair of "Iron Man" shoes that fit and were "so perfect, mum!"
A schoolmate decided to tease her about them, saying she was wearing boys' shoes. She was thoroughly confused - she came home that evening asking how they could be boys' shoes when she was wearing them and she is a girl. Girls' clothes are girls' clothes because they are being worn by a girl at the time, and the same goes for boys' clothes. That apparently is how she is fine with Daddy wearing a kilt (don't judge her response - just laugh until you fall over, like DH and I did)
That is basically how we have raised her - gender and sexuality are not primary concerns. Of course, her intellectual disability does play into it - she is almost 10, but functions at a 4 year old level.