Little information here..
Everyone.. no matter what age they are, can imagine kissing other people. And they usually do.
We are sexual beings at all ages.
Being married, in love, or dating does not stop this. And it is ok.
If you DIDNT find other people sexy/attractive, I would worry that you were a bit obsessive and might be a dangerous person to be in a relationship with.
So stop beating yourself up for feeling sexy with someone else. This is natural, and quite enjoyable.
I flirt, and I am married with a child. I flirt because I love the attention , I am a sexual being, and I enjoy feeling wanted. I look at attractive men and women, sometimes even with my husband.
A marriage/ relationship works when you can accept this fact about BOTH sides. Someone else said he ( your boyfriend) will be 'tempted' sometime too. And this is also true.
Though, I would take the word tempted out of the statement. Because I dont think there is anything wrong with flirting, dancing, fantasizing, or watching another person you find sexy. What I do see a problem with is allowing another person outside of your relationship to actually touch you.
This I think is crossing the line.
But, and I am sure you dont want to hear this, you are still young. IT IS O.K. !!!
You will break someones heart, you will have your heart broken, but life does go on. You DO get over it and so do they.
Dont fret too much about living inside of societies rules yet. You have plenty of time to do this.
But if it makes you feel bad the way you are behaving then dont do it. Dont do things that make you feel bad. Life is too short.
If what is making you feel bad is that your b/f MIGHT find out..
then dont let him find out.
Stop letting this other person touch you. But keep flirting and have fun.