It sounds like you had made up your mind in advance.
You don't tell them because you want to protect them from grief ... I know this sounds harsh, but that's the argument of cheaters. If they were ever to find out, hell will break loose, don't you think? And why? Because they would have wanted to know and would want a say in the matter because it is to some extent their right to know. One of them is as responsible for this as you are.
I don't believe in any party having more rights when we talk about a pregnancy. It may be the body of the woman during the first months that is more important, but after that time, both parents have the same responsibility. Just because you will be the first one to know what is going on, shouldn't give you the right to handle it on your own.
If you don't want a child for yourself and want to take precautionary measures to make sure you won't get one, it is the time to talk to them now.
If you would be ok with a child and won't make a step to prevent receiving it now (just take precautions from now on), then wait till it's sure that you are pregnant and tell them. There is the possibility that you aren't pregnant still, but you can't change what happened at this point in time.
None of you was up to this possibility and both (all three) of you should have a say in a matter that will affect everyone greatly for the rest of their life. That would be my take on the matter.
Good luck with handling this tough situation and making up your mind.
[Edit. Because I am missing YOUR wishes here kind of. What do you want? Another child? How important would it be if F was the father for your husband? How strong are your families mixed? Would it make a difference (like: this is my child, this is yours or our (meaning all three of you) children)? Some of the questions that came up when I asked my men about their opinion, we would be in a similar situation like you three if it were Lin to be a possible father and not Sward.]
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.
Last edited by Phy; 12-23-2011 at 08:54 AM.