Im a little long in the tooth for jelousy now but i was more on the recieving end other than the giving end.
Its not pleasant and can be very degrading and soul destroying to think the person you love could believe that you would actually cheat on them with no given reason for concern.
Anyone in those circumstance needs to realise that the problem is with their partners insecurities other than something you may have done, so the sooner you deal with it the better xxx