Really? I never picked up on this before, but then again I never did get to watch that part of the show in a format faithful to the JP version so I wouldn't be surprised if it was something they removed from the original dub.
First Yamcha was deathly afraid of girls, then he was okay but only around Bulma, then he wanted to marry her. After Namek, he enjoyed looking at sexy girls in the gym, suggesting that his fear of girls was wearing off, but not necessarily that he was ready to cheat.
Bulma never really loved Yamcha. She liked that he had a massive power level and that he wasn't as short as Krillin or green as Piccolo or as old as Master Roshi. She falls for every attractive guy in the series. She wanted
Zarbon just a few episodes ago.
Yamcha knew he was finished as soon as Vegeta moved into Bulma's backyard and started demanding that she serve him, and Yamcha was forced to admit that he couldn't compete with Vegeta physically.
Future Trunks said that he was born while Yamcha and Bulma were having a fight, and that Goku can't tell them about it or prevent it from happening, otherwise he'll never be born. And then the timeline gets changed and Yamcha and Vegeta both spend three years training to fight the androids. And Trunks is born in spite of that change.
If it happened in two radically different timelines, it was always going to happen. It's inevitable that Yamcha was going to notice other girls, and it's inevitable that Bulma was going to dump Yamcha and boink Vegeta at the drop of a hat. The idea that Yamcha cheated is just a way to make Bulma seem like less of a slutty whore.