There's a valid concern if your romantic partner is saying that they don't want you going to [location] with members of the oppisite sex; especially if one or more of them appears to be and or is rumored to be a womanizer / seductress.
With that being said though, Hinako and Asahi are NOT dating / married, so him not wanting her to go to the beach with memebers of the opposite sex seems at the very least childish and at worse possessive.