Yes, at age 17. I had just started having sex with my first girlfriend a few weeks before it happened. Before her, the only sex I'd had was secretly, with my best friend, a guy.
One week, he stayed over at my house for a few days (he got in a big argument with his dad and got thrown out). Ours was a small house and he slept in my bedroom. I promised myself I wouldn't have sex with him — but in the end, we were just WAY too attracted to each other to resist. For three days straight, I had sex with him (oral) in the wee hours of the night and then had sex with my girlfriend (vaginal and/or oral) the following evening. My girlfriend had no idea. The three of us even spent some time hanging out together, which was weird! It was probably the most blissed-out week of my life so far.