Can pot help with arousal in a couple and improve sexual relationships?
How does it affect relationships in general?

It's totally individual for each couple, I can tell you that some couples will be more passive and others will be more active.
But in most cases, pot helps a lot to improve sexual relationships.
It's great for loosening up both partners, it helps them trust each other more, and it makes them feel more strongly about their partner in those moments.
Also, there is an excellent article on the subject https://www.smokeandpoke.com/blog/can-weed-make-you-horny/, if you are interested in learning more about this and you care about this issue.
I can tell you from my own experience that pot has helped me a lot in my sexual relationship with my partner, we have become much closer.