I'm torn

Last semester I met the guy I have always wanted to be with and since we have worked through a lot of hard times and are planning to get married. BUT. There's another guy who is honestly so fucking attractive to me and I want to hook up with him just one last time( we have been together before) before getting engaged. Someone help. Am I terrible? Should I tell me boyfriend I need a week or 2 to be stupid, go to parties and get drunk, kinda like a hall pass?