Kinda like what DemonKazV said, it's not exactly an important subject. However, to still answer the question, sure, it's possible. One notable instance where it was questioned was with Alex Shepherd; once upon a time, it was even discussed on his page. I personally don't remember all the points to this argument, but I do specifically recall that there's a book I think in his room that's titled something like "Anal Sex for Dummies." It rose some questions.
I think the only protagonist you could maybe make a case for is Heather, and even then that's mostly due to the lack of relevant material pertaining to her sexuality. James, Murphy and Harry had wives, Alex and Henry had female companions which hinted at something deeper and Travis... Well, Travis is a truckie. Sorta self explanatory.
Although the subject may be an intriguing one to explore thoughtfully in any future installments, I wouldn't say that sexual orientation is important at all.
If she were a bit more balanced, Angela would probably be the first character to spring to mind, obvious reason why is obvious.