You're getting a bit off topic, but I'd say you're experiencing liberal hypocrisy.
Regardless of how you choose to label it, it sucks. Nevertheless, it certainly doesn't sound like you're working in an environment that is pervasively intolerant or hostile to you due to your sexual orientation. So, boneheaded though that one comment may have been, I'd advise you to not get too worked up about it. Consider it an insensitive but well intended piece of advice.
The reality is that simply mentioning homosexuality will make some people uncomfortable. You're own behavior (going out of your way to avoid discussing your boyfriend and only mentioning him by accident) suggests that you are well aware of this.
When you get down to it, all they were really asking you to do is what you were doing already.
We live in a time in which homosexuality is rapidly becoming more socially acceptable. However, there are still many people who are offended by it, embarrassed by it, or consider it an outright abomination. Giant retailers aren't in the business of social engineering. They want to sell to everyone, regardless of what retrograde ideas they might subscribe to. Thus, liberal or no, they want their employees to be discreet about any controversial beliefs, practices, etc they might have.
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