Definitely concur with other responses.
It's hard when you feel like "what am I doing wrong". My bf has issues with ed, all 20 years we've been together. It can be emotionally devastating.
I HIGHLY advise finding other options for you to get off and since bisexuality is in the picture-what about her being able to help you? Even possibly a strap on?
NO disrespect to him-but I've found that taking the heat off the guy entirely-can help A LOT.
If he can't finish; then let that part go and be a little selfish and start finding ways so you can orgasm without intercourse with him.
A vibrator, dildo, strap on-either of them could use it on you, in fact, both of them could take turns doing it.
The thing I see is that since there are three of you, there is the option to alter the focus. Make some of the focus about you and her sexually bonding more, instead of each of you "riding him". There is SO MUCH bonding that can happen sexually between two women that people never really think about.
Also-just because a man loses his erection, doesn't mean he's out of commission. It means his penis is.
He has hands, he has a mouth, he can do other things to show his love, affection, attraction and attentiveness to you.
Both my bf and my dh, regularly either finish or lose their erection LONG before I am satisfied. In both cases, they understand, we're not done til I am satiated too. They use toys on me, or their hands or their mouths or whatever they think up, until I am satisfied.
I HIGHLY encourage removing the PIV as being the big focus. Don't even try it for awhile. Just avoid that one altogether for a few weeks-do other things.
"Love As Thou Wilt"