To be fair, when I played Lara Croft, I just never thought of her sexuality. Whether she was interested in guys or girls never crossed my mind. I imagine many who played game felt the same way.
As for the question of will straight fans continue to play Lara Croft if they openly make her queer..........why not? Its not like the game focuses on her relationships. You'd have to be quite homophobic to avoid a game where any romance will barely be visible. So sure, Lara Croft being the first big name Lesbian video game franchise character would be a big deal and a positive improvement in the industry.
All I shall say is I'd prefer that the developer and publisher have the guts to officially have Lara Croft be gay rather than keeping things ambiguous. I can't see most male gamers caring, this isn't a romance game after all. If anything, most young guys would likely be all for Lara being a Lesbian for obvious reasons.
I imagine the most common reaction would be: "Lara is a Lesbian? Seriously? Thats HOT!"
Say whats you will about that, but at least its not going to be a rejection.