"To lie in bed" is to be under the covers. "To lie on the bed" is to be on top of the covers.

to lie in bed = someone lying to his lover while making love

Actually, I never hear "to lie in the bed." If a person is under the covers, we ususlly say, "He's in bed," and don't use the verb "lie" at all. However, if someone is tired and just wants to lie down for a while, I could say, "He went to lie down on the bed for a while." In this case, we would often drop "in the bed" and say "He went to lie down for a while." When something is very obvious Americans often drop unnecessary words. "On the bed" would probably most often be used to indicate the location of something. "Where's my coat?'' "It's on the bed."

Sorry, "helpful" people....

The coach is right. Close the question before "helpful" come to confuse you ;)