You know, those oils -- EFAs -- are good for everyone, regardless. They can be found in a cooler in any natrual foods store.
But taking some flax oil isn't going to make erection happen when it wouldn't otherwise, or sustain erection when your brain and body say erection time is done, or make erection "harder."
You don't have ED, based on your posts. Really. You haven't even gotten to the point where you're sexually active over time to even begin worrying about that, seriously.
And guess what? If you get your knickers in a twist getting all paranoid about it, chances are good you WILL have erctile difficulties. So your best bet is just to chill out about it, realize sex doesn't center around your penis anyway, and take the time you need with any partner in the future to get comfortable, trusting, and pace sex as is best for you.