Is a feeling of lust a sin?

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Recently due to a bet with some people, my friend had to lip-kiss me. From then on,I've felt loads of attraction towards him. During the bet, it was a very emotionless thing, but even though we didn't love each other we liked the kiss and frankly speaking I did want more. Even he developed a lust towards me and he wanted to kiss me again, so I just gave in. Now we kiss each other everyday when we meet. I know he isn't in love with me and I am not in love with him, its all lust. And even I feel whatever I am doing isn't the right thing and I should cut it out. But another part of me wants to do it... Is this feeling a sin? Is lust without love not right?