Alice the mermaid and Bob the human have been dating each other for quite some time, and being young adults filled with love and copious amounts of hormones the two of them want to take their relationship to the next level by engaging in intercourse. However, there's a big problem in the young lovebird's way: how can the two of them find a position to mate in that is comfortable enough for both parties to enjoy the act?
The mermaid may not be comfortable laying on her back on land (especially if she has some sort of dorsal fin) but might be little more than dead weight if she were on top, whereas the human is likely to drown if they tried mating in the water. The mermaid may be more comfortable swimming, but it would be hard for the human to focus on mating and enjoying the experience while treading water. Mating in a standing position without any support is also uncomfortable for most humans and many men report it difficult to maintain an erection or complete the act of sex in such a position (there are clinical notices about this on public health sites but I'm not linking them to keep this as SFW as possible), possibly because of muscular tension in the pelvis and legs while standing or differences in blood pressure when standing. Mating in the surf zone would likely be annoying because they would be constantly bombarded by surf and the mermaid might end up with sand in her gills.
Other parameters to the question:
- The typical mermaid problem isn't an issue here. Mermaids have male and female reproductive organs similar to dolphins (and similar attitudes on how to use them)
- The mermaid can breathe air and can survive out of water for a decent amount of time, but also has gills allowing her to breathe water
- The mermaid cannot transform her tail into a pair of human legs and walk on land that way
- The mermaid may or may not have a dorsal fin. It might end up being absent if its too much of a hassle for the story, but most marine animals do tend to have one
- The goal of the activity is for it to be mutually pleasurable for both participants. Non-penetrative or otherwise alternative methods of intercourse like oral sex would be much easier to achieve, but these two want to take it all the way.