Chazal tell us that marriage is a shmirah for a man from sinning with his eyes or worse. Yet many married men continue to struggle. Do you see this continued struggle a result of a lack of Sholom Bayis? Why isn't the shmirah of a wife sufficient for many men?

It's a shmira when it supplies him with what his soul really seeks out in sexuality, which is a closeness of husband and wife. This is only through proper intimacy. When he uses his wife to satisfy his selfish desires, he's never satisfied, 'masbio, ra'ev'.

I once heard a line from a Rav that I think nails it to the core:

"Shmiras Einayim doesn't go away for married men, quite the opposite, it becomes three dimensional."

Meaning, almost all frum from birth people weren't physically intimate with a girl before they were married. So their pleasure in looking at a girl was "2-dimensional". Not to get too graphic, but married men know what its like to be with a women physically, so when they are mistakal at a woman, it is "3-dimensional". ודו"ק

Now, how to square that with the Chazal you mentioned, I'm not entirely sure, but the starting point is that, like many statements of Chazal, it is not meant to be taken in an absolute manner, but rather in a qualified manner or as applying to only an aspect of the thing. The reality of "Pas bsalo", and having an outlet for your desires, is just as real as what I described above. 

They are both true, in some ways it easier and some aspects of it become more challenging.

yehudi1 wrote on 15 Mar 2023 17:34:

Ahhhh, the million dollar question! concernedjew21 has given a very deep answer here.

But I think the question really touches on something very deep about the human psyche: We always covet what we don't have. Why? Because a person is on this earth to be in a constant struggle with the yetzer hara and OVERCOME his struggles. 

If he would not struggle, there would be no purpose for him on this world and he would... die.

People can covet certain women, and guess what? Very often, the husbands of those women, are coveting other women. Crazy! But that's life.

Now, about the Chazal. I think what Chazal are saying is that marriage is one tool  in a person's arsenal against the yetzer hara. There many tools. Chazal say the Torah is a tool, for example. Yet, there are many b'nei Torah who struggle. 

Marriage will help a person only if he is in battle mode and very aware that the yetzer hara is around the corner. If a person is alert, marriage is a huge help. But if he's not, he will struggle like anyone else. And even he has "great" shalom bayis.

Just my thoughts, but curious to hear what others think.

Short answer - similar to what simchastorad said - marriage w/o sholom bayis cannot have real intimacy. I believe it actually makes it worse because you will be searching for other sources to fill the void and never be able to fill it. 

Longer answer based on personal experience .... I am very old to this problem with MANY years of being weighed down my P&M and general Shmiras Einayim issues. This was compounded further by my wife having some periods of time with significant health issues that made Sholom Bayis a monumental struggle. A few years ago the doctors found something that made a significant difference in my wife's health issues. Over the last 5-6 months we have worked hard on repairing our relationship and Sholom Bayis but it was a struggle to overcome many years of pain and distance. As an incredible incidence of Hashgacha Pratis, at around the same time I started listening to the Halacha Headlines podcast (partially as an effort to find better material for my downtime). Just a few weeks ago he did a show on Shmiras Einayim focused on GYE at a time where I was in the right frame of mind to actually listen.

At least for me there has been a very clear and dramatic change in just three weeks!
following GYE  => increased Sholom Bayis => better relationship with my wife in all areas => improvement in the general struggle with Shmiras Einayim (both virtual and live)

I know this is not the point of this thread, apologies, but that is so wonderful that after suffering for so long with health issues your wife was able to find something which helped her

yehudi1 wrote on 15 Mar 2023 17:34:

Marriage is a shmirah for a man from sinning but it does not heal addiction...
As Rabbi Twerski Z"L famously said:
"It is important to know that an addiction is under control before considering marriage. Marriage is not a hospital and does not cure addiction, and continuation of the addiction is likely to ruin a marriage."

yehudi1 wrote on 15 Mar 2023 17:34:

In my case the addiction lead to the breakdown in shalom bayis and ultimately the marriage

yehudi1 wrote on 15 Mar 2023 17:34:

Good question,
In short, I don't think Chazal spoke to men who watched porn or was addicted to masturbation.
Our brain desires and is hungry way more than a normal man.
Yes, shalom beis is a big game changer and having a good pleasing wife can also help, but sometimes it makes the even worse.  

Lets not forget that less = more