How so?
For example, say there are only 2 characters that are interested in a male PC, and 2 characters that are interested in the female PC. One is gay, and one is not. Unless I decide to mix up my sexual orientation/gender, I have *zero* variety in how my (lets go with male) PC gets for romance options.
Now if all the romanceable characters are bisexual, my potential options and permutations open up. Since you can only romance one character in a particular playthrough.
*If* you keep your character's gender/orientation the same in all playthroughs there'll be indeed no change in your experience in the first scenario. However, it hinges on that "if" -- if you do change, then the playthrough 1 will have _different_ character(s) show interest in you, compared to playthrough 2, etc.
In contrast, in the scenario where all the romanceable characters are bisexual, you will experience the same situation in all your playthroughs (4 characters showing interest in you) no matter whether (and how) your own character changes. While I agree the _number_ of options offered in each playthrough will be greater than in the alternative scenario, the _variety_ of these options offered by each playthrough becomes reduced, in this particular case all way down to zero.
To use DA:O as quick example, having Alistair try to hit on my character if she's a woman but not doing that if he's a man, creates two different experiences in two playthroughs of the game. Having Alistair try to hit on my character if she's a woman but *also* if he's a man, as well... only triggers a "meh Alistair, i've already seen you do that" for me, the second time it happens. Now multiply that "meh" by all the LIs, happening as early as in the 2nd game, and it becomes doubtful I'm even going to finish that second playthrough, let alone start a 3rd.
Mind you, I'm not saying the interest of LIs needs to be gender/orientation based. That's how DA:O did it, but it's just one way to go about it. If it's instead something like having all characters bisexual but paying attention to what actions and stances your character takes, and reacting accordingly, that'd be fine too as far as desired end effect i.e. variety of offered options goes.