My advice is don't do it.
I was in the same situation with my first love.
Great! She wanted to be with another girl. I thought all my birthdays had come at once and didn't need any persuading. Anyway I was cheating on her with everybody including escorts, no harm you would think?
I could not have been more wrong.
I only found out how deep my feelings were for her once we had the threesome and they developed feelings for each other.
The whole thing is messy and you need to ask yourself do you care about her?
If the answer is no and she is not your life partner I say go for it.
If the answer is yes you should be asking yourself why does she want this? Whats lacking in our relationship?
I don't think you are that sure yourself by the fact that you are asking the question on here in the first place.