Do you think it's appropriate for your fiancé to ask a girl he used to "talk" to (as in pre dating stage) for her number ?

My fiancé was low key flirting w a girl he used to talk to by replying to her Instagram story. He didn't say anything inappropriate but I know how he flirts.. well anyways he ended up asking for her number & she didn't respond yet. I feel like there is no reason to have her number. & that it was weird to ask for her number. I asked him why he did that & he said that he didn't know why & then blew up & gaslighted me which makes me think it was even weirder for him to do that. I don't know girls i felt like it was an unspoken rule out of respect for your partner that you don't talk to to old flings.. but maybe I'm just being controlling ?