How far away from RE are you? You're going to have to make the $18,000 at some point to retire, right?
If it's 1 year away, then you would be missing out on (rough calculation) $18,000 * 0.04 = $720 in interest. In this case, it'd be like paying $720 for a test drive of retirement and putting off the RE date by a summer.
If you're 10 years away, you'd be missing out on 10 years that your $18,000 would be working for you. (my calculations: $18,000 * (1.04)^10 = $26644, or $8,644 in lost interest).
On the other hand, if you want to take a break from work, and have the means to do it, I say go for it! Isn't MMM about being happy, healthy, and badass BEFORE AND AFTER retirement? You'll still get there eventually, and when you get there, you might be healthier, happier, and you will have the memory of a work-free summer with your girlfriend.
I'm facing similar thoughts myself. I'm still in the "pay off student loans" part of building my net worth... whenever I think about opportunity costs it makes it really hard to justify buying anything above the bare necessities or taking any time off work. But personally, I think there should be limits to the delayed gratification offered by saving everything for retirement. You have to find a balance that works for you, lest you show up at the gates of RE only to find yourself overworked and burnt out.