...are sexually frustrated and therefore more likely to go on shooting sprees.

I'm not sure if it makes them "more likely," but there is a pretty high correlation of that in mass killings. I'm not sure if it's the most prevalent cause, but it seems to be a popular contributing factor and part of the whole "outsider" or "marginalized" persona of many of these shooters.

...and a lack of appropriate gun control.
...and has access to guns

This is ancillary to the problem. As stated in my original comment above, mass murders are committed with a wide variety of tools, some of them not necessarily tangible. When mentally ill people have a desire to kill, they will find a way to do so, whether with a cargo truck, a knife, homemade bombs, a gun, or a shitty economic system that they force on a society.

We may disagree about the availability of certain tools, but I try to leave failed prohibition laws and policies at the door. Unless, of course, you are the target of the prohibition. :)

#banbeth