My boyfriend was like "let's go clean my room".

I don't live there but every night I bring an overnight bag and sometimes things can get thrown around. So, we were cleaning up and he asked me if I wanted to wash anything. I said no, I'm fine. Then he asks if I want to wash my underwear. I look over and They Aren't Mine! I started to say that and I was in the middle of saying it he was realized and was like "oh these are *roommates* girlfriends. And I was like "Mkay". Then we kept cleaning and when I was making sure that there were no more clothes to wash I was like "you sure there's not more girls underwear in here" and then he got annoyed and said "why are you being lame?" Then after a few minutes he was like "sorry she does laundry here" all sarcastic. First, I was half joking. and second I don't understand how they got into His laundry in his room?? Maybe I'm overreacting but I leave for work earlier than they do so they are together in the morning sometimes. Is it possible for her underwear to get in his laundry just from doing laundry in the same room? I live with 6 people (family) and we rarely get someone else's clothes in each other's laundry.

Update: We talked about it again. He still seemed defensive but I think he just didn't want to have an issue. He explained (I already knew this) that he has two hampers. One dirty and one clean. Usually he just throws his clothes on the ground so the clothes get mixed up. Him and his roommate often leave their laundry in the washer or drier so sometimes he'll leave his clothes in the washer and his roommate will put them in the drier. Or if his roommate takes his clothes out of the drier he'll put them on the couch. Because his laundry is not organized the underwear could have been clean and he just thought they were dirty because they were near other dirty clothes. I'm not sure I buy it 100% and I'm still suspicious. But, I have no proof so for now I'm going to trust him but keep an eye open. I'm not sure where the underwear went, he picked them up from in front of his roommates door and I think put them in his bathroom. Honestly, I'm not sure how much of it was true. And he might be cheating. But again, I don't have proof so I'll have to just have to be wary of stuff like that I guess.