I’ve never had a relationship but I’ve witnessed it in many of my friends and people I know! I called it the happy fat spread
Usually when someone is pregnant I notice the man gets a dad belly! Possibly cause he’s eating for two the same as the mumma or cause he’s become tuned into his partners hormones and gains a happy belly with them.
But also when couples get happy and relaxed into a relationship they sometimes put weight on like they feel comfortable and secure so they let go of trying to keep efforts to stay in shape as they don’t need to worry about finding someone
Quite funny and interesting to observe it happen from a third person perspective lol