My choice of the word “boring” was probably inappropriate and led you to focus on that single word… Of course, there are exceptions and you seem to be one of them. I never assumed that “every monogamous relationship is doomed to be boring”, I just notice that in most cases, closed binary relationships finally end up in failure. The failure is not always apparent, as there are a lot of couples who have all the external appearance of being successful, but in which the partners experience frustration, often unconsciously. They seem to be going along perfectly well, but their love has become a kind of companionship deprived of the essential, while both are thinking it’s the normal evolution of a couple as we’ve been led to believe by our brilliant leading scholars.
Now let me focus on your words “open” as in your sentence “it's really pretty hard to be bored if you are with another person that is open as well”. I totally agree if "open" means open to others and open to the love, however it happens.
What would you do if someone happen to be in true, mutual love with you or with your husband? Would you reject that person, causing her and your husband or yourself an excruciating pain leading both to wish they would die? 
What I am saying is that the "inevitable" feeling of insufficiency, lack, boredom, restlessness, lack of vibrancy, frustration, lack of passion, being in what many consider the loss of the early romantic hormones that are so desired - whatever you want to call it - is NOT necessarily because in this society is monogamous, heterosexual, or doesn't condone sexuality with children - but it is because in our society we make our decisions about our partners based upon desires that are bound to lead to such experiences.
You ask what being in love means? I ask what does choosing your partner mean? I have loved many people and some have loved me, but I choose to open certain chakras only to my husband because he is the one that I choose to have that kind of relationship with. My opening my lower chakras to someone else could be entertaining as it would be entertaining to him to do so with others - but unless we BOTH wanted to open that same energy to a third person in equal degree it would cause a riff in our energies together which we both hold as the most valuable thing our lives.
My husband has never been jealous of me showing love from my heart and upper chakras towards others, including men, because he knows that I have chosen him knowing that he is the BEST for me and he trusts my ability to gauge my own energies. I have had close male friends who I loved very much and who wished they could have a sexual relationship with me if I were available, but I wasn't. They didn't turn down the love of friendship I offered because I wouldn't have sex with them.
It sounds like what you are talking about is truly a fixation on genitalia - not on love. I hold myself free and my husband free to open up to others with tremendous love. I however hold certain energies for him alone as he does with me which allows us to have a deepening of relationship which opens up a certain kind of experience based upon a level of trust and undispersed energies that would be diluted and confused if opened up generally with others.
The only way I could "fall in love" (I am thinking you mean with sexual desire) with another person is if I allowed that person into relationship with me in a particular way. If I were to do that, then I would be giving up the very sacred and deep intimacy that I have with my husband to allow someone else in. If someone else chooses to "fall in love" with me in a way that they need/want sexual interaction with me and would not accept just my heart centered love then they would be trying to force their way in a place that is not welcome and be acting from in my view an aggressive and hostile place. I would not have to turn away their love however - just the "genitalia" and the holding of the primary place my husband has in my life.
I think it is our societies fixation with the lower chakras, the genitalia, chemistry, hormones, physical attractiveness and other lower chakra concerns that makes us choose our mates based upon those and therefore makes us get bored, frustrated - whatever you want to call it.
When I say open I meant open hearts, open minds, open psychically, open spiritually, open creatively. If all that is open is the lower chakras then we think we have to get everything from our partners, including all the love, all the feelings of belonging, oneness, losing oneself as an individual - all those things that we like to get from others when we first meet them and "fall in love" that tend to fade. When inviting a partner based upon higher chakra criteria, you don't have to worry about closing off your heart to others and you don't have to worry about getting bored or frustrated.
I think few if any people in our society make the choice of partner in this way Iguana - let alone more than one. I'm just saying that perhaps the thing to change isn't just involving more and more people with genitalia and continuing to look further and further outside oneself for lower chakra satisfaction.