Even if he spent an entire class talking about it, so what?
Right. I don't think this is that big an issue.
Kids need to learn about the world they're living in. Tom can put his fingers in his ears and go "laa laa laa" as much as he likes, but the truth is that world is now one where being gay is accepted and gay marriage is legal in many places. That might not be the world he grew up in, it's not the one I did either, but it's the world we live in now and that's what kids need to learn about.
And I note Tom continues to ignore the point that as kids start to discover their sexuality it's a lot more healthy for them to do so in a world where how they feel is accepted than one where it's thought to be wrong, shameful and acting on it is illegal. No-one consciously chooses their sexuality, to be told that how you feel - how you are - is wrong is reprehensible. I'm pretty glad that as a species we have evolved past that. Well, most of us have.
It should also be noted that this teacher didn't mince in to class actively trying to "promote the gay lifestyle" (whatever the hell that means). He wasn't handing out fliers trying to recruit more kids to gayness. The pupils asked the questions and he responded. I'm going to assume that he did so in an age appropriate way because most people would, especially a teacher. You can question whether he should have let that go on all class - if it did. I would agree that he probably shouldn't have. [As an aside, when I was doing Computer Science A Level, one of my teachers was a Star Trek nut. If you could steer the conversation towards that then that was end of lesson!

]. But, overall, will it do those kids harm? They get a better relationship with that teacher, they learned a bit more about the world. I don't think this is worth the amount of hand wringing that it has generated from certain people.
Because what those people really mean, but are trying not to say out loud, is that homosexuality is sinful and an abomination and should still be illegal. But because that is no longer the prevailing view, they have to pretend that they're trying to protect children from all these "child catcher" gays "promoting their gay lifestyle". They'd be perfectly happy with children being indoctrinated if it was to a worldview they approve of.