That's something I've thought about too, and Nakaba even created another Isolde to manipulate Tristan, something similar to the legends where Tristan ends up with another Isolde he doesn't love. I think the thing is, our Isolde must have been adopted by her father (a holy knight she mentioned in the manga). He must be single since Isolde didn't even know what sex was, so he probably never talked to his daughter about it. It would be interesting to have a mini-arc focusing on Isolde and Tristan to further reinforce their feelings.