Many years ago, when I was living in Chicago, and my brother was in school at Champaign / Urbana, he came up to visit for the afternoon. He brought a friend, and she and I hit it off quite well, so later that day I went back down-state with them to spend the weekend with her. How far have you traveled for a sure (or even potential) thing?
I travelled 1500 miles for a sure thing. It’s over a decade later and I’m still here.
Looks like about 215 miles, but that was not just for sex; it was a long-distance relationship.
I flew a Cessna over 500 kilometers for a fun night with an ex.
Good times had by all.
70 miles for a good chance something might happen. It did. Never again. 
100 miles. A friend with whom I have a mutually beneficial relationship lives in NYC.
Chicago to New York once…however far that is (I flew and it was for a long weekend).
180 miles driving for a night (Chicago to Indianapolis)…was totally worth it…I miss her.
Across the Atlantic - 4,000 miles according to MapCrow. A girl I’d been seeing at college who went home to the US. I turned her down in the end, though, because she treated me badly while I was staying. Namely: for the first week I was there, her boyfriend was too. His presence, and the fact that she’d got back with him, was something I hadn’t been told prior to my arrival…
I’ll be traveling about 1500 miles for a sure thing–from Kansas City to Boston for some hot monkey lovin’ with my husband. Does that count? 
Man I’d be pissed.
Least she could do was set you up with a friend of hers so trip wasn’t a total waste. 
Hey same as me ** jjimm**
Mind you mine was an unexpected encounter in a hotel in Tinley Park, Chicago.
Bless her little cotton socks for making an Englishman feel welcome in the US
161 miles.
3000 miles. NJ to CA
6.5 years later and it’s as good as ever.
1620 miles. Definitely worth it.
Anytime, baby. 
For the last year, my wife and I have been living about 17,000 km (11,000 miles) apart. So do our infrequent visits to each other count?
This discussion would not be complete without a mention of The Sure Thing.
Wife-to-be lived in Chicago. I lived in a southwest suburb.
We met on a cruise ship in the Caribbean. We didn’t go that far specifically for sex (ok, I did), but that’s how it worked out.
Reno, Nevada. (~450 miles)
Google comes up with 8.5 miles, but I remember taking a lot of back roads and it was after midnight and I was riding my Dad’s bicycle with the stripped right crank so it seemed farther.
I may have traveled farther later in life, but that trip sticks out.