It is established that Timmy is actively trying to avoid falling in love with Tootie, and goes so far as to say that she kissed him once and it was not voluntary. However, that kiss was on the episode "Love Struck!" (season 3, episode 24), and it completely contradicts what Timmy is saying. The kiss was voluntary because Timmy was in love with Tootie at the end of the episode - he shot himself with Cupid's arrow specifically so that he would fall in love with her. The kiss was definitely not forced upon him, and based on Cosmo's reaction to watching it, it was not a platonic kiss either. ("That's cute." "Ok, this is a little awkward." "OK, this is downright creepy."