Romances in RPGs tend to be quite optional. In the original BG you can easily play the whole game without triggering them, and you can always just say "hey...you know what? I'd rather not" to a flirty character. BG:EE is a bit more direct about it by design (especially BG2:EE, I never expected my evil cleric-wizzard egotistical gnome lady to get in a "love" triangle with a Blackguard half orc and a vampire, but the game almost insists on it), but even there you can always tell someone to buzz off.
Also, I wouldn't expect all companions to flirt with you without you flirting with them first. So if one of them is too flirty for your taste, just don't take them along? Evil companions tend to be more flirty, so assuming you will play a mostly "good" party, even if the game kinda sets the alignment system aside, avoiding that kind of company should be relatively simple. Especially since Larian games are not known to turn any and every relationship in a suggested romance.
I wouldn't oppose the toggle option, but I don't expect making your character incapable of love to be needed for any kind of playthrough.
