Let's go for it, but don't bet on it.

Undyne chuckled at earthquake's expression & backed up close to him. "You do realize I may or may not be married?" He reassured the Royal guardian. 

"Why? Who are you married to? Vivian the shadow siren?" 

"H-how did you know?!" 

"I've met her once before. We were great friends." 

Then Undyne shifted her rear end at earthquake stating him to, well, you know. 

Here goes nothing if we get sued he thought. He got up & got down to business. Earthquake sank his thing in & gave Undyne what she wanted: him. 

Basically, the whole reason Undyne & earthquake are doing this outside was because due to lioness being a lion & Toby fox being a dog (somehow, lions eat dogs), earthquake couldn't be seen with Undyne after lioness stated Toby fox being here "enemy." 

Actually earthquake got used to it quickly after he plunged himself right in. Undyne stuck her dark red tongue out & eyes closed. The satisfied ringtail cat thrusted slowly & carefully while Undyne panted in satisfaction. Both him & his teacher really enjoyed being the only ones outside and not inside. Undyne gave out deep, light moans while earthquake grunted to get himself in more deeper. 

"Even if I'd care, I'd still be able to-" 

then Undyne gritted her teeth in pain & then began her sentence before earthquake could finish his. 

"God, earthquake! You're going in too hard & painful! Slow down!" Earthquake felt total guilt until he smelt something bloody. "Wait! What the heck?!" 

"Forgot to inform you that I'm actually bleeding for right now." 

"How?!" 

"My creator roughly dog-styled me as a punishment for seeing you." 

Earthquake said he was so sorry to hear that & he continued. The more he thrusted, the more Undyne panted & moaned. Earthquake knew he couldn't resist his teacher claimful body just until earthquake felt that same wet substance again. Undyne moaned romantically and earthquake got  it out of her. Undyne fell the the ground with a romantic expression on her face. She turn d to her student and hugged him. "Thank you, my student," she said in a chilled voice. "Since alphys was taken down by your father, I felt so lonely and thought I'd stick with my favorite student, which is you, earthquake, my sweet student." Those words came at earthquake like a lovely scent about to strike his ears. He blushed and said something to her. 

"Well, you're without doubt the sweetest teacher I had so far. You've taught me everything I need to know about combat & I'm now a part of the Royal guard just like you. If your were never there, I wouldn't know where I'd be. My wife Vivian got taken by my predator mother & I felt so lonely as well. My sister Ivy is already married to knuckles, my sister raindrop now lives with her mate flowey. Undyne, my lovely teacher." 

The two kissed each other & got up. 

"C'mon. Let's go clean ourselves & get back home."