If you think logically, much like a bachelor party, it is a celebration of the end of your bachelorhood and welcoming you to your marriage. A new beginning. I like the theme because it gives this guy who has been on the show for quite a while a good closure to his singlehood. She is declaring the end of his single life and from now on, she would be his girlfriend and he would thus not be attending this show anymore.
Very logical yet she is one of the first female participants to be able to fast forward the process, journey of goal seeking (relationship) and make the guy she likes see what their future would be like. This future pacing scenario makes him think of the future that they would spend time together, this construction of a future event, if it syncs well with what he is looking for will be a strong pull.
Internally, the guy would be fighting hard, resisting and convincing himself why it cannot be done. We can see that he was interrogating her with a barrage of questions, trying to make sense of the whole matter. The innocent and optimistic girl was able to answer his questions without much thought because in a relationship , other than logic, lies a more important component - the desire to love someone. She expressed this so beautifully in this episode and was able to touch the heart of her beloved guy.
Naturally, it ended nicely with them holding hands and agreed to seeing each other and of course, this is the last party that he would be attending, a testament to the importance of a strong desire and positive outlook of their future winning over the possible volatility in their journey.