Wait, what?
It’s highly unlikely that your envelope will have any live germs or bacteria on it - if it’s fresh out of the box or enclosed in a mailing from a business it’s probably never even been touched by human hands. But it’s almost a certainty that your finger will have all kinds of microorganisms residing on it.
I marked the I lick return enveleopes, but I can’t remember when one did not have the peel and stick seal.
Same for birthday cards.
Stamps are also self-adhesive, so no licking there either.
Despite being unable to recall when I last had to do such a thing, I ticked ‘lick’.
Most DM or plastic wrapped birthday cards containing envelopes are fulfilled by machine these days - as my sister would say ‘untouched by human hands and feet’.
Bill printing and stuffing machines are used to print and stuff envelopes without any human interaction. If you get a bill in the mail with a return envelope inside, it may not be absolutely sterile, but the possibility of it containing dangerous germs is almost nil.
Lick away.
But then I’ve never gotten a paper cut on my tongue from licking my fingers.
Considering some of the places my tongue has been, return envelopes are the least of my concerns. 
I lick the envelope, the paper, the stamp, and the mailman.
Yeah, I don’t remember the last time I licked an envelope someone sent me. But that’s because of the internet, not because I’m worried about catching something from a new, untouched envelope.
I never lick the adhesive on any envelopes I send out; I don’t want the authorities getting my DNA.
I also wear gloves to avoid leaving fingerprints, and purchase envelopes in a different city, paying cash and wearing noseglasses. You can’t be too careful.
I can’t remember the last time I had to use a return envelope, but I luck an envelope if required.
I pay bills online, but in the rare instance where I need to use an envelope, I tape it shut.
I lick all kinds of things more organic, shall we say, than an envelope. I probably lick five envelopes a week between business correspondence and bills.
This! 
I use my sweaty brow since I live in South Georgia. OK, in reality, I usually just lick them. Maybe an average of slightly higher than one per month.
Does anyone else remember elementary school valentines with flavored glue on the envelopes?
I wet something instead of licking because I got a papercut on my tongue as a teenager. Never again.
Maybe you’ve learned things since then. Like acute vs obtuse angles?
Don’t live scared.
My wife pays the bills. I lick her tongue, and she licks the envelopes. It works for us, and I very much look forward to the 30th of the month.
When we do the Christmas cards I use a wet rag.
Sealed with a Kiss,
Shodan
I lick my monitor every time I pay a bill.