I've got three memories that I would call the most memorable experiences.
1) The first time a girl swallowed. I was 21 at the time and thought it was the best thing in the world. We'd go at it. At the point of climax, I'd pull out and she'd swallow it all. Sadly, the relationship side didn't last.
2) Before my wife, there was one girl who I would consider the love of my life. We dated off and on for a few years and fooled around while each of us had SOs. The last time we fucked, we were in N.Falls. I took her there for Valentines day. Actually, I skipped out on the girl in #1 to be with this one. Before sex, we started talking about having a family. She's leave her fiance and I'd leave my then-gf. She asked for me not to wear a condom during sex. Suffice it to say, I was excited. But, during the point of climax, I pulled out because I just didn't want a kid at 21. She swallowed it all and said that we could never do this again because I pulled out and she wanted a kid. I've seen her since. She's married now and still doesn't have a kid. I never bothered to ask why.
3) Back in HS, there was one day when I had sex with two girls in the same day. One was my then gf and the other was girl from #2. I had sex with the gf in the morning before class...got a bj from #2 and sex at lunch...and had sex with my gf in the late night.