Your goal is not to get a "yes" from your partner to do an action. Your goal is to create a space where you are both completely comfortable doing an action.
You should not feel forced to do anything. You are not obligated to do any action to any person.
Sometimes it is easier to ask for consent in a less stressful time when nothing is expected, just to establish boundaries.
Consent does not ruin the mood, it makes both parties feel comfortable with what is happening. So, if anything, it makes whatever is happening better. Nobody wants to feel taken advantage of or uncomfortable.
Even if you have done something in the past and it was okay then, do not assume it will always be okay in the future.
Even if consent is given beforehand you should still ask in the moment, especially if trying something new.
Any answer that is not the word "yes" is a no. "I don't know" is not a yes.