I (18M) work in a small gym and it's only me and one other staff member working at a time. Earlier today my coworker, let's call him Adam, was working at the desk in the gym, whilst I worked in the larger, sports hall that's attached to the facility. We have little contact while we work as our stations are quite far apart but I occasionally come down to visit when I get bored/have nothing left to do. Today I went down to the gym and his girlfriend was there working out while they chatted, but he soon started to get a bit handsy when she "needed help" with the weights, I soon left. I had heard rumors that they may have had sex at work before so I was quite aware that they may try it on. I continue with my work for about an hour until I go back down to the gym and see their stuff there but not them. I check the disabled toilet as that's where I thought they could be if they were getting it on. They weren't there so I checked the changing rooms and I found him locked in the toilet (it's kinda of a mini room rather than a stall) in the staff changing rooms. In an alarmed voice he says "hello" and asks what I'm doing, I state I'm checking all the doors as we had an alarm malfunction this morning (not a lie) and that we were due to lock up in about 10 minutes. I pretend to leave but wedge the door open to confirm my suspicions and sure enough, he let's out a huge moan, followed by his girlfriend giggling and the sound of him spitting. Shortly after, I hear what I assume was him giving her back shots in the toilet and then both moaning loudly. I leave and the door squeaks loudly but they didn't notice anyway. I am now incredibly uncomfortable and she stays for the entire 3 hours left of the sports hall being open (different opening times to the gym) and stays in the office flirting with him whilst I do admin work on the computer. Later on, I find a shitty excuse to separate them (he's still relatively new so i said it was "training") and ask to have a word in private. I tell him I'm not stupid and I know what he was doing earlier and they weren't very good at hiding the noise they were making. I said not only could he lose his job, but he could also lose his place in the school (the facility is tied to the school he goes to, it's a partnership) and that he mustn't do it when I'm on shift, or at all. Adam has the audacity to say well we did it away from you so that we wouldn't make you uncomfortable, to which I said that's not really the point, it's grossly inappropriate and it's unacceptable. For reference, we had a kids martial arts club going on at the same time and any one of the kids or parents could have walked in on them making love, creating a whole different world of shit for everyone involved. I said I won't raise it with my manager this time as Adam is good at his job and far better than many of his peers, although now I'm wondering whether I should have told my manager, or whether or not I should have just pretended I didn't hear. Really confused what to do now, I feel pretty violated and disturbed as I was hoping those rumours weren't true and that it was simply made up due to them being VERY comfortable with PDA. AITAH for bringing it up with him? I told him alone and I don't think his gf knows about me knowing or talking to him as he said let's pretend this didn't happen and forget about it as we left the changing room where we had the chat.

Id really appreciate any advice please, this is a really difficult situation that I'm in rn and I just feel disgusting icl.