So confused. Questioning?

[Anonymous]

Hi all, I’m new here, and I’m struggling with my sexuality, I think?

Please be warned this will be long - sorry! 
Some background info:
I’m in my late 20s and am a gay man. Always have been; it’s never come into question, never been challenged, and I’ve been incredibly fortunate my whole life, to be accepted by my family and friends. It was just always “A Thing”.
I suppose it would’ve been different if I had been more effeminate, though? I know effeminate gay men are always targeted, but I’m sort of the middle of the scale; neither too masculine or feminine. 
I’m happily married to my childhood sweetheart - we have been together in and off since we were both 14 (but became permanently “on” when we were 19, and have been happily together ever since) and are still to this day, very much in love and have a very happy and healthy relationship. During our “off” times of course we “sowed our wild oats” with other guys, but always found our way back to each other, so we have definitely had other gay experiences. 
He briefly had a girlfriend aged 13, before he came out of the closet, but I have always known I am gay. “Gold Star Gay” as it’s called…
Until now.
This is where my issue lies…

We have a female best friend, just a few years older than us. 
She is also happily married to her own husband. Again, a very happy and healthy, strong relationship. I have never seen her so happy and treated so well. She’s flourished since being with him, and he has also become our best friend too. We socialise as a group a lot and are very close. 
Of course, I’ve always known that he is very good looking and a lovely bloke - honestly my husband and I fancy him a bit lol! ?
But very recently, I have found myself having…unwanted physical feelings…for our female friend!
It started quite recently, and I know how awful this sounds, and how gross I sound even saying it, but her breast size has quite significantly increased lately as she is currently pregnant with their third child. 
I have found myself attracted to them in particular, but also to her - physically!
Honestly, I feel dirty and disgusting, guilty and ashamed. 
I’m so confused, and I’m just questioning everything.
There’s absolutely no doubt about the fact that I still love my husband and of course I am still very much attracted to men!
But I have had very inappropriate thoughts and dreams about our female friend lately, and I feel so disgusted in myself.
What really made this worse was the other evening, I popped over to return something I had borrowed from them, and her husband was in the process of giving her a back massage, due to the lower back aching she’s been having, and she obviously felt comfortable enough around me (being a gay man and how close we are!) to be entirely shirtless! I didn’t know where to look! 
I felt so embarrassed and yet so strongly aroused! I quickly made an excuse and left! That night, it really sunk in that I was attracted to her physically, and worse still, I became aroused in my sleep due to a dream I had that included the four of us including her (I’m sure you can guess!). I woke up and had to strip the bedsheets and throw them in the wash. I was mortified and so ashamed. My husband just assumed it was a normal “arousal dream” like we’ve all had before! I couldn’t tell him the truth.

They’ve asked us to meet up since and I have pretended to be ill, because I just can’t face them. I’m so embarrassed and ashamed and disgusted in myself! I’m feeling incredibly depressed about it, and like I have been/am being unfaithful to my husband. 
He knows something is wrong, but I cannot tell him. I just can’t. 
I’ve just said that my depression is flaring up again, and he believes me… it’s not exactly a lie… it IS flaring up again, but I haven’t told him why and cannot.
I know too many people who have ended up breaking up when one of them has come out as bi or pan or something. I’ve seen it happen too many times, and I just refuse to even risk losing the love of my life for the same reason. I’ve seen so many previously happy, healthy and strong relationships destroyed by it. Including this same female friend’s ex husband. I remember it absolutely devastated her when he came out as bisexual. He is now with another man too. It took her a long time to move on to her currant husband, to trust again and to allow herself to fall in love and be loved again. It was a very hard time for her because of course she and her ex still loved each other but she had to let him go. 
I don’t want that for my marriage. I don’t WANT to be attracted to women or anyone else. I love my man. I’m incredibly attracted to him and we have a fantastic sex life too! So I don’t understand why this is happening at all, let alone now? 

Sorry this has been so long and if I haven’t made sense anywhere. My head is a real mess, and I feel so lost and scrambled, like I’m losing my identity. I don’t know who or what I am anymore, much less know what to do.
Please don’t tell me to tell my husband or my friends. I literally cannot do that. I will lose one, if not all, of them.
I just want these thoughts and feelings to stop. I want to stop feeling so wrong and so dirty and ashamed, guilty and confused. 

Thanks for reading. 
- ?‍⬛