Bad idea unless you are both comfortable with everything the other person values in the relatioship and agree on all what activites are allowed and not allowed. i love 3somes. i love orgies. i love sharing. being in that type or relationship is eyeopeneing and great, but its a narrow line. and nobody i know have walked it without fucking up. (not say it cant be done, but it takes a very mature set of people who are at the same stage sexually, mentally, and emotionally.)
then there are the power issues. 3somes are rarely equal. Both of you have to be ok with starting as a team and one accidently getting left behind or left out.
If there has to be rule setting or compromises against key desires, really dont do it. example= telling a person who loves to suck "no lips below the shoulders" is bullshit and meant to fail. If youre that uncomfortable, dont bother. you will lose.
why...
lots of people get carried away or drunk or mad or stupid or jealous. and rules get broken.
i went through the exact same situation as you. I love women, am much more physically responcive to women, but i love to be fucked with real dick and wanted a male spare to switch out with a female.
the female spare was wonderful. i got to pick and i landed a hotti with personality and passion. the male.... never really happened.
to make a long story short, i felt used. everytime i would hook up with the girl the feeling got worse. then i felt cheated because he could be hers, but i could only be his.
it sucked ass major.
So unless your standards in the relationship match exactly, dont do it. avoiding bad spares is only half the problem. the other half is protecting the santitiy in your relationship.