I need your opinion and advice.
I recently broke up with my boyfriend of 5 years because he cheated on me and didnt want to break a contact with the girl that he cheated with. Now I actually think that he wasnt with her only once as he said but many times. I know now for sure that he is with her now and she is like his girlfriend. He went into her arms soon after I left him (moved out of our flat) and I know this because I was spying on him. I also know that she is very much in love with him and she loves him, but he still wants to be with me. He is sending me text messages all the time, calling me and sending me emails. He even sent me flowers to my work. I am sure she wouldnt like this if she knew this. But the fact is that he is lying to both of us. I know that they are together now, but he doesnt know that I know this and he is just saying that he is not going to have anybody for a long time. Also he is very jealous when I tell him that I am going to meet my friend, as he thinks that I am dating somebody else. He is such a liar and he is playing these games with me well I am not stupid and I know everything whats going on and I am never going to go back to him, although he wants me to. Before when I was with him, he was all the time hiding texts and phone calls from this girl (I know now that he was really cheating on me with her and it wasnt only once) and now when he is with her I am sure he is doing the same thing, hiding to her that he is sending me text messages and that he calls me.
The thing is that I dont want this girl to be with him, I dont want her to be happy with him and I just want her to be a bit suspicious and jealous. I want her to have doubts in him, because he is not a good person and he doesnt deserve somebody to love him like that.
I never want to be his girlfriend again because I know that he is never going to change and he is always going to lie about one thing or another thats how he is, but if he agrees I just want to meet him for sex which would mean that he is cheating on his current girlfriend. She is actually going to go abroad for a while to study, but she will be coming back to visit him all the time and then after she finishes her study (after a year) she is going to stay with him. Well thats the plan for her.

Do you think is it good idea to have sex with my ex, while I am single and until I find somebody else? Or is it bad idea? I would kind of revenge to that girl who went with him in the first place although she knew that he had a girlfriend.

most of all for you. Dont do it, he is not worth it, if you just want sex for now, plenty of other guys just for that purpose, but that way you will at least start the moving on process and revenge is not worth it, she is already with a cheater who unfortunately will hurt her, since its just his nature. Revenge will just hurt you...so, girl, make every effort of moving on, my advice