Can guys learn to self suck? Yes.
Is it learned through practice? Yes, for most people.
Is it a matter of flexibility? Yes, for the most part. (unless you have about a 10+ inch penis).
Can anyone do it? I dont see why not.
As for myself, I actually learned I could do it while I was in high school. I dont remember how, but I somehow came across a porn site of a gay guy who was notorious for his self-sucking movies. And I somehow ended up on a journal thing that he wrote that actually gave instructions on how to learn to self-suck.
Basically for the biggest details, if you cant do it yet, and want to learn how, follow these instructions:
--Practice without any clothing (especially any clothing that has belts, etc.) as these get in the way.
--Stretch your back and neck very well before attempting this, as its quite possible to seriously injure yourself, especially when youre first beginning.
Its best to lay near something tall, like a wall or a desk, and put your legs up high on the wall or object. Lay with your back and head on the ground, and slowly slide your upper body towards the wall (as your feet move upwards along the wall). Once your shoulders are near the wall, slowly bring your legs down over your head, and sit in this position. --Important-- Let gravity do the work here... dont force your legs downward, as this could severely hurt your spine or neck. Also, dont stay in this position for longer than 5 minutes at a time. Allow time to rest in between sessions.
If youre not a particularly flexible person, this is about as far as you should go for a week or two. Once you feel you can do this without any pain or forcing, try to get your feet to touch the ground behind your head first. Then push them back until your knees can be on the ground behind your head. From there, its all a matter of breathing out, and learning to bend at your lower back.
From that position, anyone with an average sized penis or bigger should be able to self-suck without problem. After doing it often, your body becomes a lot more adapted to it, and it becomes a lot easier.
One important thing to note, though, is that this isnt meant to work in a matter of days. It took me about 2 months before I could do it (and I consider myself to be more flexible than most people).