The think the objection is that this is specifically targeted at women, and it's something that someone might be self conscious about.
"Free coffee to shortest/tallest/skinniest/fattest man" would be also be offensive IMHO, because it's singling out people for a trait for which they maybe don't want to be singled out.
Crude humor is great, if all parties are in on the joke; I believe the point that parent was making was that all parties are not necessarily in on the joke.
I don't know how to say this, but...you have extremely uncommon use-cases:
Many people listen to music on stereos and don't necessarily want a device plugged in, so
either doesn't work or is substantially less convenient than e.g. casting from a phone.
Not hating on your setup at all, but it's very niche, in my experience.