What is the biggest ‘acceptable’ age gap in a relationship?

Everyone will have a different opinion. It also depends on the age of the youngest one. If it's a teenager, I'd say only a few years is acceptable as an age gap. Though in the end, love is love and it's not defined by age. Sometimes age is just a number. Some people could be okay with a gap of maximum 5 years, while others would be okay with a gap of 10 years or even more. For me personally, someone who is 10 years older would be my absolute limit. More than that just seems like too much of an age gap for me. It'll be different for everyone. It all depends on personal preference and a personal point of view.

I think for minors it is 2 years, and no sexual actions going on. If the male is 18, and the girl is 16 or younger, then he could be put in jail if pressed. Other than that, I think at most 10.

It depends how old you are. If you are both over 18 and mature then I don't think there's an issue. If one of you is under 18 then keep it within a few years because the transition from high school to work/college can result in a very big gap in lifestyle.
My aunt and uncle are 14 years apart and got married when my aunt was in her early 20's and they are really good together. Age is just a number though.

I've always been taught that you were suppose to half your age and then add 7. Let's use some examples, I'm 15 so I half that to 7.5 and then add 7 which is 14.5. Meaning that I can have a relationship with someone who is between 14.5 and my age preferably. If you were 50, you'd half that to 25, then add 7 = 32. So you can see that it manages itself. Although if you were 100 you could date a 57 year old, but by that time your libido will be down the drain. Here's more info on it: http://www.scienceofrelationships.com/home/2011/10/4/the-dating-equation-your-age-7.html