Well, you could consider yourself bi or you could consider yourself not. You said the relationship is purely physical, do you find yourself physically attracted to other women? Bi can mean anything from being emotionally and physically attracted to both sexes, to being physically AND emotionally attracted to one sex and only physically OR emotionally attracted to the other, or any combination of the two.

Honestly, you don't have to label yourself anything if you don't want to. However, I understand the feeling of wanting to label your sexuality so you feel like you know that aspect of who you are.

Here's what I would do personally. If you don't find yourself attracted to other women, just her, and have never liked any other women in your life, I wouldn't necessarily consider myself bi. Alot of straight people are attracted to someone of the same sex at some point in their lifetime, but it doesn't mean they aren't straight. That doesn't mean you can't accurately label yourself bi, if you feel like that fits you.
My last bit of advice would be to think of this: if you know for sure you like men (which I'm assuming from the bit about occasional boyfriends) think of this. Compare if you like women the same way you like men. Do you feel the same towards them emotionally, physically? Can you see yourself in a relationship with a woman (other than the current one)? Things like that. Good luck.

Sorry I couldn't be more helpful, but with things like this only you can truly figure it out.