Why did Steve react to Robin's secret like a 2019 teenager?

Initially I thought the same thing as you. His reaction was curiously understated.

But, then, once you've been threatened with disfigurement, beaten and drugged to within an inch of your life, and knowing that somewhere out there demonic monsters are about to be unleashed, suddenly learning that someone is gay is probably not gonna register on your 'give a sh*t' scale.

Not everyone in small town Indiana is Mike Pence. Thankfully. Just like today it was the conservative religious folks who felt strongly about these things, moderately or non religious ones not so much.

But I agree with the previous poster. Steve's had a really, really bad day first of all. He probably expected to die down there when the Russians were through with him. But even more than that, the things he's experienced starting in season 1 would have changed his perspective. Ask the average 19-year-old what they worry about, evil alien monsters and the end of the world probably won't figure in to the conversation. When you have big things on your plate stressing out over the small stuff seems kinda silly.

Because they were friends and it was just the 2 of them therefore there was no reason to be a jerk to seem cool in front of his friend group.

I think he acted a little let down at first because he had just put himself out there. Then, he didn't want to make a big deal out of it in front of her because they were obviously close at this point (nearly getting killed with someone makes you tight like that). There was a lot more going on in his (the character's) head than he let on, I would imagine.

This is hilarious! Steve is a good guy at the very basic level. He may not have started out as one back in Season 1.

People are acting like gays and lesbians didn't exist in the 80's and they did. In fact it was during that time that they (we) began to start feeling more comfortable with ourselves. Granted not everyone was coming out...but they did come out back then and YES, there were people who were okay with it...like Steve.

I found Steve's reaction a perfect fit for him.

He's sort of been established as a stereotype, but every aspect of that stereotype has been knocked dow. And he's also incredibly loyal to those he has suffered with a.k.a his kids. This is someone who still struggles with the idea of being 'cool' even after high school has ended, yet he's still on good terms with his ex-girlfriend, who cheated on him, and is bffs with her little brother's nerdy friend.