Some by-law infractions can carry demerit points, and ones that prohibit turns are, unfortunately, ones that do. However, the municipalities aren't always bang-on with reporting such by-law infractions to MTO, so there's a good chance it will just go underneath their radar screen. If the officer had charged you with Proceed Contrary to Sign at Intersection under the Highway Traffic Act, section 144 (9), there would definitely be demerit points involved.
Your insurance company cares about HTA convictions, not demerit points, unless your demerit points reach the point where you'd get your licence suspended... if you're concerned about that.
If I were in your situation, I'd probably just pay it and let the chips fall as they may. $18.75, no driving conviction on record is better than $110, guaranteed 2 demerit points and a guaranteed conviction on your record. However, if you're really concerned about demerit points, you could plea-bargain to another municipal charge, such as disobeying an official sign, if you wanted to completely eliminate the possibility of the points. (Be careful to ensure that it's a municipal disobey sign and not HTA disobey sign.)