Someone in 1994 took out a patent on the use of minoxidil for causing erections. Here’s the LINK. The person taking out the patent claims that it is effective as a topic medicine (on the skin). I don’t recommend trying this, but it would be interesting if it works, because this is an easily available product.
This might work because minoxidil raises nitric oxide levels.
Once again – information only available on my site!