Again, I will note that bisexual people do not tell people they've met, even if they've started to become friendly to them, or even if they start dating them, that they have slept with both men and women in the past. In general, people don't discuss past sexual history with new relationships, and bisexual people are no different from any other kind of people.
I get what people are saying, "why don't the bisexual people discuss their bisexual nature/history before I start flirting with them or they start flirting with me," but bear in mind, the two 'straights' don't really discuss it in DAO either.
Yes, we do find out ol' Al is a virgin. But Morrigan kind of clues us in that she has slept with other men before (at various points), but you'll note she never really chooses to discuss it that much with you. Most people would rather not go into detail about the sexual pasts of their real-world LIs, which is why most of them also never bring it up. And bi people are no different.
BTW, one other thing about Isabela. I personally think her lusty promiscuity is her version of commitment-phobia. As you find out in her romance, she had a husband, (so there was a point in her life where she didn't mind being "tied down"), but he died tragically, and that has made her "gun shy" about getting too involved with anybody. In some ways, she's just chosen ever since to pursue one night stands, so her life will not again be complicated by a commitment that could end up hurting her again. Well that, and she's afraid getting too involved with anybody (man or woman) will keep her from her life of freedom at the helm of her big boat on the sea. As with many characters, I think some of her swagger hides an inner vulnerability (and that's exactly what pursuing the romance to completion enables you to learn about her).