I like writing stories in my free time for my friends to read or just for fun because it makes me happy. I have written only a few with sex scenes in my life so I'm not an expert on this. But these are just some things I can say from my experience.
One thing that makes sex scenes difficult and a pain to write is that you want to make it realistic without too much details (unless it's some sort of lustful romance novel, which I've never been very interested in writing). People want their sex scenes to be just right: Not too cheesy but not too horny. Writers usually want it to have meaning and impact the reader to a deeper level than simply turning them on or grossing them out. Finding that balance is not easy.
I would say that another reason writing this is difficult is because sex is something private and personal. Most societies have this standard where sex is something that people don't freely and flatly talk about in public. People often use phrases or euphemisms like "making a home run" that simply imply something sexual without explicitly saying it. That's why touching on this topic is sensitive and almost a taboo to some. It's also uncomfortable that once others read your story, people will finally know specifically what kind of kinkiness, sexual, and/or horny thoughts that comes to your mind. Talk about awkward...
The only way I get through this is when I stop thinking about what other people would think and just write it out. Writing is partly for the readers, but it's also for yourself. Just think of yourself and no one else. Write what makes you happy without thinking what would make other people happy. Once you have everything out, then you can be all critical about it and start considering what your readers might think when you edit that section.
This is probably the most embarrassing response I have and will ever answer, so don't judge too harshly. I hoped this helped a bit.