Are there doctors who are not married? Just curious. If yes then i see no reason why single men who are in fact mature enough to be able to join and respond to calls if needed.
And on another note, most calls meantion who is involved in the call, whether a male or female so no excuse. Like you said when the opposite gender is needing help you don't respond and for same gender calls you can be there and help out.
So what really what it comes down to with these organizations is how many people they have, and if they have a lot of willing members then they"ll start narrowing it down to their ideal criteria. So if they can have what they want, they"ll narrow it down... Married, 25 etc...
Now for chaeverim they really should take more single guys in my opinion - issue is many area they consider themselves some sort of police force and are kinda losing sight of their bread and butter. Lockouts, boosts and helping people who ran out of fuel are nearly as exciting as running after a burglar with lights and sirens or other police like activities.