I had the thought that sometimes girls will entertain being flirted with for the ego boost despite being with someone.

Yes.

Also, if you're in a marriage or closed relationship, it's something you can do too in order to maintain an abundance mentality without cheating. The MRP community calls it "catch and release". Take it as far as you're comfortable but also know you can score, and then end the interaction.

I did love helping her shop, it felt fun and playful.

Just look at it as practice.

So where did I go wrong?

I wouldn't necessarily say you "went wrong" anywhere. Some guys might have tried to get past the "I have a boyfriend" thing, but I always had a similar approach to Rollo's for that: even if it's a lie to be a gentle rejection, I don't want to waste any more time on someone who isn't in to me. And if it's the truth, I don't want to knowingly mess with a cheater. As far as making your intentions known sooner, that can help, but I think you had finesse. You just picked a target who was taken.

The only thing I might have done differently back when I was single and picking up chicks was look for signs of interest, especially clusters. For example, did she make eye contact, smile, and best yet preen in some way? Then go for it. However, if she didn't show interest, I wouldn't bother.

I had just gotten off pretty late, I needed a few things for dinner the next night. My plan was to make my grandparents dinner and dessert the next night. Walk in to Aldi and I see a short, petite, brunette looking at fish. She looked like a young Claire Forlani with mild acne, incredible ass. Initially, she seems like she's grazing aimlessly, though she sticks around for a while at the salmon. What a coincidence; I'd like to see her fish. I walk up to her "trouble with the salmon?" is my opener. She laughs, and I help her pick out her dinner. We talk about past and future travels, about future dreams and goals, about best and worst jobs, I flirt and tease throughout. We pick up the salmon, along with potatoes. She helps me pick out my couple items. A solid ten minute interaction. Finally, as we have checked out, I go for the close: "Hey, we should grab coffee or a drink soon". I would have gone for the instant date but we both had refrigerated items, I live an hour from the town I was in. She says "I should have probably told you a while ago, I have a boyfriend"...

So where did I go wrong? Well first, I didn't establish intent with my opener or strong enough throughout, though I didn't because it's a smaller town and I don't want to become "that guy" in a town I work in. To me, although I was not explicit, it would seem obvious based on my flirting my intent. Though I'm starting to think I should have established intent anyways a la "Hey, I only help out cute girls with their shopping". Thoughts? Would establishing intent have been worth the risk? I did love helping her shop, it felt fun and playful. That felt like good game, I just don't love that I helped her shop for her date night with Chad lmao. I had the thought that sometimes girls will entertain being flirted with for the ego boost despite being with someone.