This guy is not much of a friend if he spends his time spreading gossip about your parents. If I were you, the first thing I would do is call him out over this issue, and then ostracise him completely. Such childish and spiteful behaviour makes him unworthy to be considered any kind of friend.
As far as gossip is concerned, don't worry yourself about it. The things that people tittle tattle about today are quickly forgotten tomorrow, when some other rumour or innuendo starts spreading around about someone else. Quite apart from anything else, there's nothing you can do about the things that have been said.
Stop and think for a moment about this whole situation. Your mum and dad obviously still love each other very much, and do not feel as if they have to hide the way they feel from you. They are only behaving in a natural way; the way that two people who love each other often behave when they feel safe and secure. Rather than being shocked or embarrassed about their liberated attitude, you should consider that this is the ideal way to behave when you are in a loving relationship or married. Sex is not something to be hidden as if it was somehow nasty, dirty, or distasteful. Sex between two people who love each other is the ultimate expression of that love, and if the children witnessed her parents exhibiting such beautiful emotions with each other, then it should be something to be admired, not hidden away.
I grew up in an environment where sex is a perfectly natural activity. It was not something hidden away, but something to be enjoyed. I have never grown up with any hangups or inhibitions about sex, and perhaps this is as a result of my upbringing. I can tell you now, seeing sex in all its wonder and enjoyment is far more exciting than watching even the most graphic pornography. One thing that pornography never portrays, and there is the loving emotions of people who really care about each other; and that is what makes the difference between having sex and making love. Your mum and dad make love with each other, and the more they do it, the better their relationship is, and the stronger the bond of love between them grows. Try to remember that there will be a point in their lives when they are no longer able to enjoy this form of expressing their love for each other, so please don't feel embarrassed or shy if you see them doing what comes naturally to them. If they are blessed with a long life, I will at least be able to look back, when they are old and grey, and have a little chuckle when they think of the things they shared together. Such memories keep love alive, long after their strength has been lost.