Retreat.

Beth was frozen for a moment, until Tim laughed softly. "Hey, relax. I'm just making a joke to lighten the mood. Come on... lets get back topside and leave these two to their activities."
Beth released a breath that she didn't realize she had been holding. "Yeah..." Once they were back on deck, outside the cabin, she turned to face Tim. "I'm so sorry, I don't know what came over me down there....I... I'm so embarrassed."
"You've got nothing to be embarrassed about, it's ok. Your secret's safe with me." Beth sat down hard on the padded bench, and Tim handed her a beer from the cooler, which she gladly accepted. Tim continued. "My brother and his girlfriend, well they like to have sex on the beach, and I'm not really into watching my brother fuck, so I swam back here. Just my luck that my bed on the boat is... occupied." Tim sat across from her, and she turned to look at the beach. Sure enough, she could just make out the moving figures on a towel a ways down the beach.
"Jesus... why does everyone have to be fucking right now, except me?"
"We can fix that, you know?" She turned to look at him with a gaping mouth, until he laughed again. "Sorry, guess I'm not funny today. I've got a big mouth, it's something everyone complains about once they've spent any time with me."
"Ugh...It's not your fault...." she drank the beer he had given her and filled him in on some of her problems with Brian, and how she was getting frustrated with the lack of a satisfying sex life.
"...so maybe that's why I'm.... playing with myself watching Lindsay get fucked..." Strangely, it seemed less awkward to say out loud now.
"That's some heavy shit. I'm sorry to hear that. If you were my fiancee, I would never let your needs go unmet like that."
"He's not doing it on purpose, he's trying to provide for us long-term. It's just temporary frustration..." I hope...
"I'm sure he's a great guy, but you've obviously got some serious needs if you're resorting to... what I saw."
"We're getting married. I have to take the long view... it'll get better once he makes partner." She drank some more, to hide her own uncertainty. Would it?
"Well, like I said, I won't tell anyone what I saw."
"Lindsay knows I was watching. She likes that shit, and she's always trying to get me to push the boundaries. She... she doesn't like Brian."
"That's kind of toxic." Tim shook his head in disbelief.
"Well, he's my fiancee and she's my best friend in the world. I can't get rid of either of them, so... this is how it is."
They talked for a little while longer, and Tim mostly just listened to her venting and offered little advice, which was good because she didn't want any.
"You know, you're a good listener, Tim. And I don't think you have a big mouth." she smiled, putting a hand on his knee. "Thanks for... helping me keep my head screwed on straight."
"I mean, when it comes to screwing and head, I'm your guy."
"Christ... I hope you just came up with that one."
"Top of the dome, scout's honor."
They chatted for a few minutes about nothing important, until Jake and Lindsay emerged onto the deck of the boat. Lindsay had a smug smile for Beth, and Jake was trying to look like he hadn't just been fucking her. Tim and Jake headed back to the beach to gather their stuff: it was getting time to head back. Which left her alone with Lindsay.
"I...." Beth didn't know what to say as she sat with Lindsay on the rear deck of the boat.
"Did you enjoy the show?" Lindsay's tone was playful without being mocking.
"I don't know what possessed me..."
"It's called a sex drive, girl."
"It was inappropriate." Beth was trying to apologize, and Lindsay was acting like it was no big deal.
"You watch porn, don't you?"
"A little, I guess..."
"This wasn't any different." Lindsay shrugged. "I'm glad I could help you get a little voyeur action on. You know I don't mind an audience."
"Can we just...get on with the rest of our day and pretend that didn't happen?"
Lindsay laughed, clearly amused. "Sure, babygirl. Whatever you want."