Is a urinary tract infection sexually transmitted?

Yes, yes: Can very well be. There is an entity called honey moon cystitis which mainly is a UTI due to frequent sexual relations. Of course sexual hygiene cannot be emphasized enough. Also, there are other reasons for UTI including after catheter placement or medical procedures or any local irritation and in post menopausal women that could be related to atrophic vaginitis changes.

UTI no, STD yes: No if ou mean bladder or kidney infections, because it would mean bacteria travelling from one bladder down the urthra which is not intact with partner's urethra. Honeymoon or post intercourse cystitis occurs only in women and is due to bacteria already around the woman's urthra being pumped up her urethra from transmitted penile thrusts. Urethritis, closest to exterior uti, is site for stds.