Praise be to Allaah.
It is not haraam, rather, it is permissible, because it is a part of the enjoyment which Allaah has allowed. Allaah says (interpretation of the meaning):
"Those who guard their chastity (.e., [private parts, from illegal sexual acts)
Except from their wives or (the captives and slaves) that their right hands possess, - for them, they are free from blame." [al-Mu’minoon 23:5-6]