Being straight myself I would personally go for it as long as I've firmly established that this was not going beyond my limits and I think it wasn't going to jeopardise the friendship.
Don’t take this as a criticism or challenge, but I find your statement to be a bit puzzling.
If you are OK with having a JO session with a friend, why would you want to spoil that environment by first outlining what makes
you special.
After all the purpose of this adventure is to help both you and the friend
discover a unique level of bonding and personal enjoyment. All you need to tell the other guy is “hell yes”. Lets do it.
If you find things should be going beyond your comfort level, then you have any number of positive ways of backing off should the situation arise. That is what you are there to do is learn about yourself and your friend.
It is your attitude and behavior during the JO session that will either cripple the friendship or advance it. If your plan is to go all “preachy” before or during a JO session, you have already harmed (likely killed) the friendship.
Having a JO session with another guy has a lot in common with going out drinking together except it is more powerful. The passions that come up while JO with someone will often loosen lips and free an individual to share their inner thoughts & desires.
It is the role of each guy to express his self, appreciate what the other person confides, and to
not be (negatively) judgmental.
You know what it feels like to have someone you admire tell you that you are looking good and doing great. Those are normal positive expressions. They are not “gay” and can work just fine during a JO session without loosing your man card.