Your reaction is far from crazy.  It's normal.  You have invested in this marriage and bonded with your husband. Now he is telling you he wants the marriage to change and he wants to seek a relationship with another person.  It is good that you are thinking about the risk of STIs and the logistics of how it could even be possible to work a relationship like this (the Valentine's problem).  So you are reacting appropriately with both your heart (emotions) and your head (your logic). 
    You did not sign up for a marriage with three people in it.  You even specifically asked your boyfriend before you married him whether he would be satisfied in a monogamous relationship with you and he said yes. Now he wants to change the terms of your marriage, by means of a demand: "you have four months."  (I have to ask: What's behind this implied threat/ultimatum?  Divorce?  He'll find a boyfriend anyway?)  
   You were honest from the beginning.  You told your then boyfriend what your values were and what your boundaries were:  "I want a monogamous relationship."  You asked if he could live with that.  He said yes, but then started in to weaken your resolve and shift your boundaries.  All that "joking" was him, softening you up (manipulating you), testing your reaction ("How strong are her boundaries?  What can I get away with?")  First he lied to you about being ok with monogamy, and finally he made an frontal assault on your values and boundary and issued his threat/ultimatum. 
    I agree with Julian Stone: you want a monogamous marriage (the normal kind).  Your spouse does not.   Your values are not compatible.  Now you have to act to protect your values.  Is his getting a boyfriend--or a series of them--acceptable to you?  Is this the marriage you want?  Is this kind of arrangement one into which you want to have a child?  From what you say above, the answer is no.  
   
  Edited to add:
    If this were a dating situation, we would be talking about "consent."  You would have stated to the man what you were comfortable with (given consent for "this" and not "that").  If he didn't respect your boundaries, and began pressuring you, or had the attitude that "'no' means 'yes'", or started "joking" about what he wanted and kept it up throughout the date ("You're going to have sex with me, right?")  and then went ahead and forced sex on you anyway, or threatened you in order to get you to comply ("If you don't give in, I'll leave you by the side of the road and you can walk home"), we would all have no hesitation condemning--and naming--his behavior.  
   It's no different in your case.  He's ignoring your boundaries and acting like an entitled asshole.  He doesn't care what you want, only what he wants, and he's going to demand you agree--or else.  I don't know what he thinks the "or else" is, but it seems like divorce.  Take a page from B'rer Rabbit--he of "throw me in that briar patch"--and take back your power.  Because divorce is not a threat to you--it's the only way you are going to be able to live a life consistent with your values.  "So you'll divorce me if I don't agree to your having a boyfriend/fucking men on the side?  Well, I'll divorce you if you do.  In fact, as you don't seem to respect my values or share them, I'm divorcing you anyway."

Last edited by OutofHisCloset (February 19, 2021 8:21 am)

I’m faced with a similar situation. My husband and I have been together for over 20 years though and I just recently discovered he’s been unfaithful at different times over the years we’ve been married. He’s stopped everything and we are going to therapy now. Therapy might be a good idea for you too. Since my whole world has been turned upside down I’m seeing a therapist that specializes in infidelity trauma, and we use her as a couples counselor as well. My husband is about to pick a sex therapist to help him find his truth. He says he’s bisexual but we haven’t been intimate in 3.5 years until just recently. So our dilemma is getting the bottom of that all, is it our connection, has his sexual orientation changed etc.,,which you may want to contemplate as well....sexual orientation CAN change over the course of someone’s life. Especially in bisexuals. My husband always enjoyed me/woman more the majority of his life (except in highschool he had strong urges and a boyfriend) but now all he fantasizes about and wants are men. We are now currently contemplating a threesome in the future..(but need to find our reconnection first) ...cause like you...I cannot picture being in a relationship where my husband gets to have sex with someone else. The first therapist we went to suggested I read a book called “The Ethical Slut”, we ordered it but I have t read it yet. It’s all about poly relationships and how you get to mold them the way you want, getting over jealousy etc...if you want to read it it could help. I’m currently not in a place where I want to though because my husband and I need to do a lot of couples work and he needs to figure out his current sexual orientation. In the end, you have to listen to your heart and soul, don’t force yourself into something that doesn’t feel right.

- Epiphany

Upside wrote:

In many cases, an affair may already be occurring, with the timeline acting as retroactive absolution for their actions. Be careful and vigilant. I say this not to scare you, but as a friend who wants you to be safe.

This was my gut reaction, too. The arbitrary timeline is such a strange detail. I stumbled across a quote once...something along the lines of "If your partner asks for an open relationship, there's a good chance you're already in one."  

You will always be enough for the right person.

Last edited by Julian_Stone (February 19, 2021 1:52 pm)

Epiphany wrote:

....sexual orientation CAN change over the course of someone’s life.

Hi Epiphany, can you tell me why you believe this?  

can you imagine waking up one day and fancying women instead of men?