Hello there, and welcome, things are not so bad it seems... you just have some stuff to understand maybe before proceeding. First of all sex gives us a rush all our own and makes us have those loving feelings. When we have sex with people we love it adds to that, which is why people who are in love want to have sex. Perhaps this guy that you "helped out" felt this way when he had sex with you. It was his first time after all and those feelings and the feelings of having got through something that was perceived as holding him back might be overwhelming him into thinking the whole situation was more that it really was.
So now what? Well, you could tell him this and talk to him about your other man and what is happening with him. Straight up, honest and open communication is always the best in my books, as long as it's done with compassion, respect and is passing on information that is beneficial. You don't need to go into details about the love you feel for this other man but tell him that you have strong feelings that you are exploring.
It is likely that this man will recoil and either disappear because he has seen that you are taken and that is that or you could spend the time explaining to him that you are able to love more than one and that there is a possibility that you might have more than a friendship but you would hope he would explore other avenues as you will be too. Its up to you to figure out what you want first, and then present it to him.... I suggest that you let him sit on it for awhile... tell him that you realize it takes time to figure these things out and that you will check in with him later about it.
Good luck to you... I'm glad you recognize that your integrity is important. In this day and age it seems there is not much thought to ones integrity.
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