That still makes me pretty mad to this day. If I could pick just one thing to ask BioWare to not do ever again in a romance, it would be having a any bisexual character hide their romantic past that they have had same gender from the PC. Anders hiding his past relationship with Karl from a female Hawke was really a poor call on the part of the writers.
I'm going to disagree with you there. On two counts. First Anders description of his relationship with Karl to the male Hawk is the very definition of pansexuality. Nothing ambiguous at all about it. The ambiguous bisexual romances are the two elves. - who never make any reference to their own sexuality.
Moreover, Anders failure to mention his sexual past to female Hawke strikes me as completely realistic. Anders is attracted to Hawke from the very beginning. He wants to have "a shot" so to speak with Hawke. Telling a guy that you have a romantic history with guys serves to check the water and see what the other guy's feelings are in regard to homosexuality in general, as well as a subtle way to start figuring out if the guy in front of you is attracted to guys at all. It's a "technique" I see employed all the time, especially if you may have to worry about the other guy turning hostile if you insinuate he might be gay (which, let's face it is always an option IRL, if not in Thedas. LGBT people are still the minority most likely to be assaulted, according to FBI reports).
When hitting on a woman, that same information might serve the exact opposite result. Women, for the most part, and with some exceptions, are less likely to respond to your advances, if they know you have a sexual/romantic history with guys. Some may just write you off as a potential partner, others might even turn hostile. I have bisexual friends, and very few of them will reveal their being bisexual to a woman until they have been to a few dates at least. And Anders' telling Hawke about his relationship with Karl comes VERY early in their relationship. It is what? Their second conversation? And Anders have not even yet expressed his interest in Hawke. Why would he do something that would quite possibly ruin his chances right from the start?
Now, it is possible to find fault with the writers for not adding another conversation about Karl with female Hawke later in the game, but IIRC the topic of Karl is never brought up again, and perhaps this conversation would have just been an expenditure of resources they didn't have. You can head-cannon that Anders told female Hawke about it, or not. Presumably not every conversation they had over their (at least) 7 years relationship is in the game.