It's different, but accepting a sexual relationship will not lock you out of love relationship.
To put it simply, sexual relationship is where you and the girl has done sexual things but not yet confess the love. If you reject the sexual relationship, you can still get into the love relationship later on when they hint or confess their feelings for you. But if you reject the love relationship, then you will be locked out of romance from them.
This relationship mechanics exist because I'm planning to make multiple endings for the characters. There will be harem ending, true love ending (where she's the only relationship you have), and friend ending (where she's not part of your harem). But I'll add an option to break up with any of the relationship you have before the story ends so players don't have to do multiple playthrough to see all the ending.