In answer to the OP's question. Of course straight people can enjoy gay-themed films. Why couldn't they?
That's like saying an adult can't watch a childrens cartonn or drama and like it because they are adults. My sister won't listen to Sinatra, Tony Bennet, Doris Day etc as that's old people's music based purely on the premise that's what our parents and grandparents listened too and liked and thus it's old and I'm not supposed to like it.
That is as stupid as saying that a straight person can't watch gay drama or comedies or vice versa as it's not meant for them.
The themes remain the same regardless. One person fancies anther but doesn't know how to approach them. that is an universal theme and mot sexual orientation specific.
If the story is about a person being discriminated against because they are gay that is no different to the same film being built od discrimination for anything else. the theme remains the same.
If you want to buy it then buy it. As someone else suggested buy it online of it's that much of a problem. Chances are you won't find it on the shelves in your local store and have to order it anyway so you may as well buy it online and save yourself some money.
I hadn't heard of the film but after watching the trailer on Youtube I quite fancy the film as I thought it was funny in an this is ridiculous way. I smiled and smirked more than a few times watching that. It looks like a good no-brainer movie and far better than anything the desperately unfunny Gervais and Russle Brand have ever come up with.