As a father of 3 sons...you definitely talk to your partner if that partner is living with you and is actively part of the kids lives as a "parent".
Regardless of the partner/parent issues, the parents MUST talk to the kids.
This isn't about some religiously/politically based notion of "right vs wrong" or the "shame of sex being dirty " or the "body is naughty"...nothing like that.
Didn't we all live through some version of this?
(If you had to "come out" as a gay man, you at some point in your childhood were told that men don't sleep with men and men don't kiss men, and that men only marry women....don't burden the kids with that religious and politically motivated crap...They will get enough of that from the media and society..)
Instead, give the kids the sex talk, with lots of information.
Let them, ENCOURAGE THEM, to ask questions..and don't make anything off limits for questions. Use simple illustrations, or anatomical dolls, or paper and crayons...but communicate! Obviously they must know that there are consequences in society for certain sexual acts, their are legal prohibitions and laws that govern some acts and relations. They need to know the mechanics of sex, the anatomy of sex, diseases and problems of bad hygiene and irresponsible sex...what molestation, incest and rape are, etc...
Children are doing what kids have historically always done...they "explore and test"....they need to know more information, it is how we as a species learn. If the parents and schools aren't providing the information and the chance to ask their wildest questions, they will talk to their peers, or the internet or compare notes in person with their friends and playmates, etc.
Responsible parenting is not easy, comfortable nor simple. It is a challenge. As a parent, you have to step into difficult situations and be the advocate, protector, teacher, disciplinarian and role model for your kids.......Is it any wonder most parents abdicate these roles to the government and public "hero's"? This is not so much a "gay" question, as it is a "parenting" question....If the parents are not that strong or well versed in their parenting skills, then the parents need to admit that weakness and consult trusted professional help, from teachers, doctors, psychologists, etc.
As a personal example...with the advent of the internet, porn has become almost ubiquitous...
Over the years, I have walked in on each of my sons at different times, with one hand surfing the web and the other hand in his pants masturbating....(sound like any one here?..LOL!) The boys were embarrassed of course, as was I. I would quietly walk out of the room and later had a conversation with each of them privately about what was going on. We would have an information and Q/A session....not a real comfortable situation for either of us but it was the responsible parenting thing to do.....NOW after all of that, my sons all know that they can talk to me and ask me anything....to this day, my 25 year old and 18 year old sons will still ask me sexual questions....the 15 year old is a bit more shy...
If this thread is a "real" situation or not. is not all that important....being a mature and responsible parent and handling it appropriately IS important for ANY parent...gay straight, biological, adoptive, foster...etc..