I think it's more to represent how intense it is and how they're not used to sensation because they (usually) never had hot lesbian sex before? Or something like that I guess. Or authors just have no clue how to write a realistic sex scene, but that can't be it.
I suppose it's the latter.
Though there are some mighty cool mangakas/manwha/webtoon creators who can pull off an intense lesbian sex scene without making it appear like it's painful even if that was their first lesbian sex ever. I think of Pulse, WDtFS and the likes.