Here's my husband's and my success story with being "monogamish." We've been together 12 years and married nine. He's 44 and I'm 36. Basically our sex drives are very different, but that's the only thing in our marriage that isn't a great fit, so it's hard for me to imagine choosing not to marry him because I'm not the porn star he wishes I was. Essentially, we're monogamish because I want my husband to be happy and I'm not threatened by him having sex with other women because 1. it's just sex (I mean, who cares?) and 2. I'm pretty positive that there isn't another girl on this earth who he would leave me for. He has helped build this confidence in myself and our marriage by being super-attracted to me all these years. If it were up to him, he'd rather not fuck anyone but me. And as much as I'm GGG, I just don't desire sex enough to want to do it all the time like he does.
Early on I was pretty horny but that fell by the wayside in my late twenties, moreso after we had two children (now 6 and 3). I'm pretty happy with sex a few times a month. We have sex more than that because I want my husband to be happy and he's madly in love with me and my body and having sex with me makes him happy. My husband is highly sexual, loves women, loves porn, loves sex. He would have a three-way with a guy or girl if I wanted to (I'm interested eventually, but not there yet), would probably hook up with a guy in that scenario just to see what it was like, and is desperate for me to see all my desires fulfilled by finding some other guy outside our marriage to get me all hot and horny again (not there yet either—though I've looked). I'm stoked at the possibility of these things, but I'm just not feeling them because I'm totally satisfied by my husband and my work and my children. Yet, I feel pretty sad that I can't be what my husband would really love me to be. And he deserves it. He's a great husband and a great father. I, on the other hand, am perfectly fulfilled by intellectual pursuits and would rather read a great book than have sex for the most part (but I'll never tell him that).
So anyway, we established a few rules...
1) Always use a condom. 2) Tell me anything I want to know about it afterward. 3) Be kind to those you fuck (don't lead them on, don't fuck vulnerable women, etc.) 4) Don't let it interfere with our family life.
He's not the straying type, and now that he's over 40 he doesn't think he's hot shit anymore (even though he is still very much hot shit!), so it took a while for him to find someone. I told him before he did it that I wasn't sure how much I would want to know, but it turns out I was curious to hear him tell me the details (more because it made him happy to share rather than it being a turn on for me). I reassured him that all was well and that I just wanted him to be happy. Since then he became interested in fucking an ex-girlfriend who lives in Boston so I bought him a plane ticket to go there one weekend, then she came up and visited us. She and I are now Facebook friends. She thinks we have a lovely family, etc., and he tells me that she would love to hook up with both my husband and I, but I've not been interested in that thus far.
Long story short: everything's worked out great as far as the whole "monagamish" thing goes. I've told two girlfirends, both of whom asked a lot of questions but aren't in the kind of marriages that would sustain such actions. My husband has told one or two friends, one of whom has subsequently opened up his marriage—at his wife's request!
We definitely still have our issues about sex (him wanting it all the time, me being unenthusiastic in bed at times). In fact, I wanted to write earlier and let you know that your "easy fix" to the sex drive discrepancy being to open up the marriage sounds all well and good, but even that hasn't worked perfectly for us because ultimately my husband would rather not stray, but would rather have me be the porn star he really wants. So being monogamish hasn't solved that entirely, but it's helped a bit and that's been good. Interestingly, the idea sort of shocks people still. I talked to a guy in a bar about it the other night and he seemed totally uncomfortable and when I asked him what the big deal was, he just kept replying that "it wouldn't work for us" but couldn't really give me any details as to why. In any event, I don't think anyone would ever suspect that we had an open relationship (getting back to another point you make), so here's just one example of two people who do and are more than happy about it.