I think there's 9 people who know for sure that me and my second cousin are together (and many more who assume and talk about us behind our backs), and they've had varying reactions.
The first person I told was my first cousin on my dad's side (my boyfriend and I are related on my mom's side), and he was like, "Well that's kinda weird, but if it makes you happy then you do your thing."
Then I told my best friend, who was shocked, but not because we're genetically related, but because he thinks my cousin is unattractive. He then told me about a teenage fling he had with his first cousin.
Next I told another friend. He was surprised but very supportive. He told me about an aunt he had a crush on.
Then another friend, who was like "Well that's weird, but whatever floats your boat."
Then we had a little get together with a friend I hadn't hung out with in a while. We ended up watching a movie and the boyfriend and I got cuddly and affectionate. We were both under the impression that the friend was aware of our genetic similarities. Turns out he wasn't. A few days later he asked "So, are you guys related?" I replied "Yeah, we're second cousins." Needless to say, him and his buddy were very freaked out. He didn't talk to us for a few months, but once he started talking to us again he never said anything about it.
Next was a friend I'd had for several years. She was aware of the fact he's my cousin, and we thought she had picked up on us being together and was fine with it since she's very open-minded. Apparently not. I don't recall what was said, but some comment was made referencing me and him, she didn't understand, and I clarified by saying that we were in a committed relationship. She got awkward and left. We didn't talk for a while, but once we did it was like nothing ever happened.
We have two friends who are a married couple who we thought would outright reject us if they found out. A few months ago the wife asked me "Is that guy you're always with your boyfriend?" I didn't say yes or no, all I said was "He's my cousin." She apologized. I told her everyone assumes we're together. Then about a month ago I was at their house and the husband had a friend over. The friend claimed he had met me and my boyfriend before and asked what my boyfriend's name was. I didn't know what to say. Luckily I didn't have to say anything, because the husband blurted out my cousin/boyfriend's name. I didn't disagree. The next week I went over to hang out with the wife. At some point in the conversation she made a comment about "people around here being too inbred to care about biochemistry". I was offended, but I didn't say anything. Later in the conversation she started talking about how wonderfully intelligent Einstein was and how the world needs more scientists like him. So I brought up the fact that Einstein's parents were first cousins, and that he married his first cousin. She was like "You're right. I forgot about that! So why didn't your boyfriend come with you?" So yeah she knows. I never straight up told them, but they figured it out, and they seem fine with it.
Whoops, that got really long! Sorry!