Sounds like deal.

Guy said:
Patchouli-sama... I can't hold back anymore...

Now, being a Korean with good knowledge of internet memes in Korea, I'm extremely tempted to edit that translation to I CANNOT HOLD! for the sake of it.

Patchouli said:
If you ejaculate like this, understand that you can never again return from Gensokyo.

... In other words, "If you have sex with me, take responsibility and marry me!"

I'm not sure that counts as a "deal", because in a "deal", you have to trade away something in return for something good, but this is just gaining something wonderful in return for gaining something wonderful.

So... how is this seiken densetsu?