Really? Platonic lunch dates with a coworker are inappropriate just because your spouse/partner happens to take issue with it? In that case, the spouse's/partner's jealousy sounds more inappropriate than the lunch.

In that case, why bother saying who you went to lunch with? Just say, "Hey, I had an amazing turkey club at lunch today." (of course, this is just how I'd prefer to handle it...I'm not insinuating that this is how everyone should do it)