I'll try to keep the spoilers to a minimum here, but there will be a vague spoiler. I'm not going to go into detail on a a specific companion in the game because that will lead to spoilers for a prior moment in the game. Again, neither are bad spoilers by any means, but if you don't want to know any kind of spoiler like me, you'll understand why I am not going into great detail here.
I don't think you can miss the offer to get this achievement, though I could be wrong (I am 99% sure I am right though). If you're chose a female as your character, you can still get this achievement. Morrigan will still offer you the ritual. There are two other members whom she can perform this ritual with (one being more vague than the other but both unlikely in the grand scheme of things). So just agree to Morrigan's offer and you will pop this achievement.
Additional note: If you don't want this to be your ending, just save prior to agreeing to Morrigan's request. Once you complete the game, reload your save prior to speaking with Morrigan and decline her offer.