How can I (M 22) avoid acting weird around my brother's (M 20) new boyfriend?

I'm a straight guy and my brother came out as bisexual about 1 year ago, but he was with a woman at the time so I didn't think I'd have to face this situation this quickly, as awful as it may sound. He broke up with her 3 months afterwards and started dating this guy around 6 months ago. They've been together ever since, but since we live in different cities, we only see eachother on occasion. Next week is our dad's birthday and we're both flying home, bringing our S.O's, as usual, meaning I'll have to meet his boyfriend. First off, I grew up quite traditional (white, somewhat religious, suburban, middle class), and while I no longer consider myself religious, I still find it quite weird seeing same sex couples on the street (I'm in college, so, yeah, not such a rare sight). Multiply this by about 100 and you'll get the situation I'll be in in a few days. Now, I don't really know the dude he's dating, I've never seen him, but judging by his FB and Instagram, he seems quite flamboyant and invested in the gay community. I was wondering how I could avoid acting weird around them, seeing as how I love my brother and wouldn't wanna sever ties with him over this. It took my parents 1 month to accept that their son was dating another man, and now they seem mostly ok with it, but I imagine they're going through the same thing. I'm sorry if this isn't the place to post such things, I just didn't know where else to do so.
*tl;dr: my brother came out as bi, is now dating a guy and he's bringing him over to our dad's birthday. I'm afraid I'll do something to ruin my relationship with him. *


Source.

For sure don't do this....

That movie was crazyyyy

@semisweet: What movie was it? I just randomly found that creepy ass gif