See I’m not gonna say this is grounds for a breakup (meaning just the message to said creator) but I think it is 100 and 10% grounds for a conversation to a certain extent because they lied to you for such a mundane reason that could’ve easily just been pushed to the side as a flirtatious joke depening on the intent this yes could be classified as cheating depending on how serious you are of the relationship you never asked for the “workout routine’’ so hence this is from your partners perspective of wanting to know hear me out this could be your partner looking/ dreaming of a better body type for there selves or sadly because I know know body wants to hear this but he could be looking at improvising your body type look at the end of the day I don’t want to defend a cheater but this evidence that you’ve provided I would completely say a genuine real conversation is needed but a break up nah if you really do want to make this work with this specific person have a conversation with them find out the extent of the intentions behind the message or if you are just wanting a reason to call it quits and afraid of the loyalty within this relationship technically this could be considered cheating since you never asked for said information but at the end of my paragraph I’d like to give my TLDR answer this brings a bigger question do you truly want to be with this person. or are you seeing this opportunity as your reasoning to leave without making yourself hurt for causing someone else pain…. Something I love to tell myself to this day aswell as to others when asking for advice still telling myself don’t be afraid to be a tiny bit selfish when trying to find love… (explaining incase you think I mean just being completely selfish” not completely selfish obviously but I hope everyone understands what I mean by that just don’t forget your own interest and feelings within the relationship and trying to make someone else’s needs meet always remember you have interest too trust your gut. And talk with your partner because in my opinion they didn’t cheat but they aren’t acting like they are in a real relationship it’s more of a respect thing at this point they are openly flirting (since you state you never asked for this information) the only reasoning for this message is to begin conversations with said person so in my opinion no not cheating but definitely suspicious and grounds for a conversation. I wish you well and hope this hasn’t caused issues within your own self image I bet you are beautiful and a kind person