Why do I feel jealous about my boyfriend masturbating next to me?

I don’t know why you feel jealous about this activity, but my guess is that you would like him to direct this energy toward you more often.

If you would like him to be more romantic/sexual toward you, ask him for that attention. (“Honey, I would love it if you would initiate romance/sex with me more often. Can we make a schedule for regular lovemaking?”)

If you would prefer him not masturbating right next to you, ask him to take the activity to another room where he can have privacy.

I would strongly discourage you from trying to shame him for this, however, unless you want him to lie about it and hide it from you. It’s his body, and instituting some non-kinky, unilateral and involuntary rule about what he can do with it won’t work out well. I’m sure you wouldn’t want him telling you what you can or cannot do with your body.

If he’s masturbating as a way of avoiding physical contact/emotional connection with you, you both need to find out what that’s about and rectify the situation. If you’re getting all the sex and romance you want, don’t take it personally. Some people in relationships have mismatched libidos and this may be his way of dealing with the situation. My guess, however, is that occasionally he likes to please himself and that this is not any sort of commentary about you or the status of your relationship. Plenty of people in relationships with more than satisfactory partnered sex lives still like to masturbate.