Good thread of wisdom and warnings. Definitely can't resist the urge to add my two cents, though.
1. What Do You Want Out of Your Marriage?
In a perfect world everyone would have this stuff figured out ahead of time and use it as their guiding beacon, but we don't live in Candyland, so I will tell you the bedrock of this thread's discussion:
people need/want different things out of marriage and/or romantic partners. This can cover big topics-- kids? dogs vs cats? where do you want to live?-- but it also extends to bigger but more obfuscated concerns. Do you want someone who does everything to make you happy? Do you want someone who will always tell the truth, even if it causes short- or mid-term problems? Do you want someone who comforts you, or someone who silently supports, or someone who gives you space? And, perhaps most critically, which things are crucial and which are "nice, but it won't kill me to do without"?
Basically, half of this topic (not the half you directly asked for, but probably the half that needs to be addressed first) is whether you two should even be sticking it out in the first place. Do you give each other what you need? What you want? Are your expectations/demands/hopes reasonable, or do either (both?) of you need reality checks? In short, can you two fulfill each other?
2. Is a "Hall Pass" A Good Idea?
I still think 1. is the more pressing answer, but you asked this, so: it depends! I honestly cannot even begin to enumerate all the possibilities, so I'm just going to say it's a fantastic, super-hot, loving idea for some people, and the world's biggest disaster for others, and various shades of gray for the rest. But for you? Basically it comes down to what and when triggers your/your husband's jealousy and/or guilt. If you can't even imagine him smooching someone else without problems, no no no no NO no. If he can't even fantasize about some other hot chick without feeling guilty about it, NOOOO no no no. I'm going to guess from your tone that you're feeling conflict somewhere, either that you're not being "appropriately" jealous (if so: PSSSSSSHT hahahahahah no such thing, go enjoy yourself) or that you don't actually want to go through with granting passes. If it's the latter, you two need to sit down and ascertain what the problem really, truly is, and then take steps to fix it. I can think of plenty of dissatisfaction that might manifest itself as "can I go sleep with other people" when much easier, accurate solutions are hovering right in front of you. Don't flail at the first thing that shows up! It really could be he's regretting his lack of opportunity for STDs-- it could also be that his friends do a great job of talking up hookup sex until it sounds like some amazing, life-enriching pursuit. (Hint: it is for some people. It's definitely NOT for others.) Or that he's bored in bed, or he's too chicken to say "I don't want to be married anymore" or, I don't know, suffering from indigestion.
Anyways. Point is: you need to think, and then you two need to talk. Probably repeatedly. Luckily, you're married, so that hopefully isn't anything new.
I wish you luck, however it goes.
Just make him, as Saturo says, get tested afterward.
Because ultimately, we can't decide this for you. Think long and hard about it, and you'll eventually figure it out 
I do trust him though, always have even when he slipped up but god, i'll never act the same way around him knowing that he messed with some other person, its not something you forget, no matter how many years go by. I dont think i'll ever forget his slip ups.
