Sean Evans' ex, Melissa Stratton, says he dumped her once the world caught wind that he was dating a porn star -- something she says he was cool with at first ... but changed his mind later on.

We talked to the model/adult film actress Friday in L.A. just ahead of a podcast she was heading into at the No Jumper store -- more on that later -- and our photog chopped it up with her to get the story on what exactly happened between her and the "Hot Ones" host ... who's quite private.

According to Melissa ... things were going swimmingly between her and Sean for at least a couple months, and she says they were having a great time together as a new couple.

We even asked her about the Super Bowl -- where she was posting tons of pics of him that weekend -- and MS says they were actually going around introducing each other to friends at that point ... which makes the split all the more sucky, 'cause she says she liked him.

As for why things fell apart ... Melissa says her work in the porn biz was ultimately to blame -- and more specifically, how her being in that industry would affect his image.

The way she tells it ... he had a little bit of a flip-flop in opinion over whether he was actually okay with her past and current line of work. Melissa claims he was on board at the beginning -- but suggests that once their relationship fell into the public eye ... he got skittish, which she gets and has no hard feelings about.

Sean, of course, hasn't said crap about it all ... and his team hasn't gotten back to us either.