What should I do?

Ok so I have had this friend for 10 years now. And he was always clearly not interested and I didnt like him either. Like looks wise yeah on both parts, but we are like brother and sister. Now he comes to me tonight and asked me to have sex so we can be eachothers first time . And to be honest I would happy its him and all because I know him so long and I can trust him, but im scared about our friendship. If we do, and start to go out, after we have sex, then what if our relationship gets ruined due to our own stupidity. I honestly want my first time to be with him, but I just am afraid to what it will lead to. I dont wanna lose our friendship but I do want it to be him. What would you do in my position? I do like him, but I dont want to ruin our relationship.