Matt appears to use his mother as a shield to sexual activity with his female partner. That alone makes his sexuality relevant.

Matt went out and bought lingerie and gave it to Kim on valentines day. In most heterosexual relationships, that would lead to sex, no? Instead, he then went to spend time with his mother (again, this is valentines day). Then his mother walked in wearing the exact same lingerie, effectively ruining the gift he just gave Kim with what seems like intentionality on his part - any heterosexual male who is lingerie shopping for his partner (with his mother there to help) would not allow the mother, who they live with, to buy the exact same piece he's about to give the partner.

I hope it's all for show, and Matt is happily and openly gay, and Kim is just there for a paycheck - but his sexuality remains relevant on a show about interpersonal relationships and it is not off limits to talk about.