UTIs can be caused by a lot of different things, mostly wiping one's butt from back to front instead of front to back. UTIs happen a lot because of how one's body is made, I've never had one for example and am 63 and have done my share of masturbating :-) But by all means, talk to your ob/gyn about it and get more reassurance that you did not cause the UTI, it is not some kind of "punishment" for masturbating.