Protip: If you're running Windows and you haven't turned off / unchecked Fast Shutdown yet, then shutting down doesn't actually fully restart your computer from scratch when you turn it back on. Google it.
Lesbians have enjoyable sex all the time, and a penis typically isn't involved at all! If yours won't behave, just treat it like it's no big deal, and continue to have fun using other parts (your mouth and fingers come right to mind), and make sure everyone involved has a good time anyway.
With a new partner, right before it's time to get down with them for the first time, it's a good idea to ask if they've ever been with anyone with ED, and if not, just explain real quick that:
If it misbehaves, it's no big deal; you're still having a good time, you don't want to stop, and you'll just use other parts to make sure they end up satisfied in the end.
No matter what, it's not about them, it's a physical issue. It's exactly like how they no matter how much they want to, they simply can't can't lift and throw a car. It's a physical limitation, not a desire problem.
If you end up having to do some DIY at the end, their participation isn't required, but certainly would be welcomed. [Which in this case means you jacking off once you've gotten them off, and having them help you in some way like making out with you, riding your face while you do it, rub you all over, etc, whatever would enhance that for you.]
Not the Speak & Spell. You leave that one alone.