Terry,

Sorry for the extreme delay in responding!

Unless that divorced person has received a declaration of nullity (an annulment), all their former spouses have died, or has some other (very rare) arrangement (such as the Petrine Privilege or the Pauline Privilege), the presumption is that those marriages are valid, and to have intercourse would be to commit adultery.

The ideal solution to this is to remain continent and reconcile to God through the sacrament of Confession, abstaining from intercourse until the situation is rectified. If one is able to, but obstinately refuses to do this, then they should refrain from going to Communion.

Someone who is willing to do the right thing, but struggles with their faith and obedience, a certain level of accompaniment is called for. A person in this situation should really consult their parish priest for help with this matter.

Eric