For another, you don't have to fret about coming up with new and interesting positions for the guys to do it in, and new and interesting ways of describing those positions. For a third, not everyone likes to write explicit sex. Still, when I write, I like to include sex scenes. For several reasons. When you're dealing with two people who are getting emotionally involved and physically attracted, it's a pretty safe bet that sooner or later, they're going to have sex with each other.
Sex is a major part of a beginning relationship.
Sex scenes aren't necessarily speed bumps in the narrative. The people who are having sex are still themselves with the same problems, interests, and attitudes towards each other that they had on the previous page. What people do and say in bed has a lot to do with who they are. Ideally, sex scenes should work as part of the story, giving deeper insights into the characters' psyche. A writer can convey a lot in a sex scene, through gestures, body language, what the characters do and don't do, notice and don't notice, that can be difficult to explain in other ways.