It may be unwholesome, indeed, if the people are offended or embarrassed by it. Or, it may heighten the pleasure of the sexual relationship between
them, if neither finds it objectionable.
I know that if you are single it is a sin
I don't know. At least some good scholarship says that the Song of Solomon begins when the "Shulamite" and the man are in courtship, and this is "wooing". Whether we "say things", or not, clearly our body language and our facial gestures do, even before we're married, so I'm not sure I want
to legalize about "saying" being sin.
This has to come under the category of "meat sacrificed to idols".
Just something to think about, I'm not trying to encourage anyone to do something which would violate his conscience. I, for one, have a real hard time with "when" the "line" comes. I'm not a "black and white" kind of guy, and not very legalistic. I try to more look for the general intent than the "letter" of it.
It is clear to me, that in marriage, if both the husband and the wife like it, there's no sin in it.
I'm also not one to be all hot and bothered by words. The words "God forbid" used by the Apostle Paul and "sons of snakes" and "child of hell" said by the Lord Jesus Christ (who was indeed sinless, so this terminology in itself cannot be sinful) would probably be considered worthy of sackcloth and renting one's shirt in many christian circles. However, some of the most blasphemous, utter curses can be said with complete politeness.