Having to keep an important part of your life secret, especially a loving relationship, is not just inconvenient, it's bad for the relationship and possibly even bad for your health (http://www.psychologytoday.com/blog/...-relationships
I don't think honesty is too much to ask. It's not fair to you or him and especially not to the other women... for example, they likely think that they're living a very low-risk lifestyle for stds because they're only with one person who's only with them... when really that's not the case. They deserve full disclosure so they can make informed choices for themselves. I'm not trying to use scare tactics with the std thing and I'm sure everyone's playing safe, but to me it's a very important point... everyone should get the chance to decide what level of risk they're willing to take on. And that's not even touching the emotional explosion that you could get sucked into if/when the truth comes out... do you really want to have that sort of drama lurking in the background of your life, waiting to pounce?
Maybe putting your foot down will give him the impetus to do what's best for everyone involved. And if he's really not ready now, maybe you two could get back together when he is. The friendship thing is entirely up to you, but you deserve better in a primary partner.