Pussy sucking may be harmful for you in two regards. One that if its not in hygenic condition then it may harm you with infection
. The second one is due to the fact that licking is super exciting for any female up there, it may reduce the pleasure and sensation while having sex. Its like when we don't touch a place in our body, it is more sensitive and if used again and again, the sensitivity decreases. For this reason it might be harmful for you in future that you may not be able to bring her to orgasm/or same leve of satisfaction with orgasm without your tongue so take due care for that, once in a while it can be considered good but not always.
By washing its actually a benefit, and no harm accrues to either her or you.