switching romances in ME2 and ME3

I'm in the middle of ME2 and I want to decide whether or not to romance somebody there and then switch in ME3. I'm gay, and I know you can only be gay in ME3, but I kinda wanna romance one of the girls in ME2 just to see what it's like, and also as a back up in case I end up not liking the gay options. I looked it up and I was told there weren't really any negative consequences, but that you wouldn't get an achievement or something? I like getting my achievements if I can, and I dont wanna mess it up. So I was wondering if that's true, and how that specific achievement works— does it only mess up if you get with someone and then break up? Or do other factors apply? Thanks in advance!