Would this be a deal breaker for you?

[ThisChapter]

Your partner texting someone they used to hook up with but hadn’t been in a relationship with?

Yes

More context needed.

When did they last hook up?

Are they actual friends and if so, how long is the friendship?

Depends on what the texts say but I suppose if there is no real reason for them to be messaging then I'd be suspicious.

Ohhh I would be wanting more information i.e like people have said was it a while ago, are they friends.

Do you trust your partner?
Are you able to have an open conversation about this with them?

Depends on the nature of the texts. DH still talks to an old FB but they play the same pc games so it's all rather boring to me, but about their hobby.

Depends on context. If that was ages ago and they're genuine friends now, talking about innocent stuff then that is different to if the hookups were more recent and the text content is sketchy.

Depends

We're they good friends that occasionally hooked up, or people that only got together to hook up?

If the first, I would be fine with it. If they had wanted to be together, they would be, and their relationship is primarily a friendship.

If the second, that would be odd and I would think there is more likely to be something more to it.

Because they were selling something on fb or randomly? Context is everything. But gut instinct? No

I think I mean yes

100% yes. They have no business to be doing that tbh while your dating them