It’s possible that your lover will want to withdraw from you if they are cheating. It’s hard to say how a cheater will behave. It’s possible they will feel guilty and not be able to have sex with you because they are considering the sex they are having elsewhere. They may also be unable to have sex with you without thinking of the person they are cheating with. A decrease in sex is not a definitive indicator of infidelity, but it may be a sign that there is something going on.