From what I understand, your situation is really not easy. You are with your boyfriend for the last five years, and you really love him. But, you saw that he answered ads on the net for one night stand with other man, and you think he is cheating on you with other men and women.
My first question would be : are you still happy with that man ? Do you think that you have a future with that person ? Otherwise than this problem, do you have a good relationship ? If you, it might be a good time to reconsider this relationship. Although it is hurting a lot, you have to think about yourself too, and staying in a relationship with someone who is lying to you will not make you happy. And, it doesn’t seem to be a recent problem, because you wrote that is has been five years.
On the other hand, if you think that the relationship is still bringing you a lot of good things, that you still have a connection, that you have projects, you can consider options : would you have a boyfriend that is bisexual ? Because from what you write, it might be the case of your current boyfriend. If yes, you don’t matter having a boyfriend that is bisexual, you can see if you would tolerate to have an open relationship. If you don’t want to have an open relationship, you have to be very clear about it ; and if he cannot respect you on that, you might reconsider this relationship.
But if you are open to live an open relationship, you can also consider this option. Some people are not happy if they have only one sexual partner. It can be what your boyfriend is experiencing.
Anyway, I think that it is the time for you both to have a good discussion. If you want, you can consult a couple’s therapist, like a psychologist or a sexologist, to help you in this process.
Thanks for writing,
François, for AlterHeros.