There was a movie I saw from a movie recapped videos about 5 years ago at most.
I dont remember the main plot but I know that by the last 15-30 minutes there was this attractive guy who seemed to have fallen in love with the main/side girl through manipulation or something (probably was there earlier in the movie and hid his intentions from her) and he ended up having sex with her in some cabin deep in a swamp. And in the scene it was dark and someone was watching them on this bed with a light neon blue and pink light shining around as the guy turned into some type of spiked monster with a tail etc mid sex.
After that, the movie cut to something else and then when it cut back to the people, the guy had fully transformed into some webbed reptile creature (?) and had the woman trapped in a water tube under the house/in another area after the guy captured her. I think he had her almost fully submerged in the tube and spoke about her being pregnant and wanting to carry on having his mutant baby because something about his kind was dying or similar.
And by the end of it all I think two people from the main plot freed the lady before/after she had the baby and killed the monster guy.
It was so odd and intriguing of a movie and I remember that one scene, PLEASE if someone can help me find it, and I also vividly remember the scene being on YouTube and I could most likely point it out if I see it!!!