Am i bisexual?

I'm female, i'm 18 and i've been with my boyfriend a year and a half... I know i'm definitely into guys, but lately it's been on my mind... could i like girls too? In the past i've had female friends, and felt the urge to kiss them... i mean i kissed a girl once, it was one of those 'quick pecks' - that girls sometimes do when they're best friends... for photos and stuff, i don't know why they do it... but hey ho... anyway, i've always wondered how it would feel to go a little further... i once suggested a threesome to my boyfriend, and to be honest, i think i liked the idea a little more than he did... We've never gone through with the idea, because A. We don't know anyone willing and B. We're both jealous people (he wants me to his self and vice versa)... I've watched porn before, and the lesbian category is pretty much the only one i really like... - Here's the stupid part everyone will probably laugh at... I could imagine myself doing 'stuff' with a girl... but never in an actual relationship, it's not that i'm embarrassed... i'm just not sure yet... maybe if i experienced it, i might like it and be open about it... or i might not like it at all... I don't know, am i just going through an experimental phase or what?