Some Fathers have in their mind that they will first see their daughter kiss a boy after they say 'I do' and he puts a ring on her finger....that's how unprepared we men can be for our daughter's sexual maturity.
As you read all this...it is firm, in order to be Kind.
When you break the Golden Rule, you need to mend it.
You broke it the Golden Rule: Treat others as you would be treated.
Suppose you came home and found him 'locking lips' in the living room ?
That would not be cool, because it could trip up huge emotional energy for you, for which you would not be prepared. It could be disturbing. That is why Hotels have 'do not disturb' signs. They are for the guests AND the staff.
Basically, you did not make any agreements with your father about making out with your boyfriend in the home that you both share.
I think your actions, while probably fun and pleasurable, were selfish and disrespectful.
You do deserve your own life AND you are responsible for the consequences of your actions and omissions.
If you can see your way to agree that you made a mistake and disrespected your father, There are 4 or 5 steps to make things right, it is the basic format of an authentic apology and an offer of how you think you can rebuild trust.
0. Ask you father what he thought and how he felt about walking in on you. Tell him you want to know how basically how it affected him, so you can understand the results of your behavior. If you already know where he stands (as in, he told you, and you believe what he said), then you can skip this.
1. Say to the person you disrespected/harmed: "This is what I did: <explain your actions on that day>"
2. Say "This is how it affected you, Dad: < explain what you understand is the impact of your actions on your father>"
3. Say "That is not the kind of relationship I want to have with you, <explain the kind of relationship you want to have, hopefully one of respect and mutual concern>"
4. Say "This is what I will do so that I NOT will re-create the same kind of impact on you as before: < explain how you will coordinate or set up agreements on what you can do with your BF and where and when>"
Simplified:
1. I did something
2. what i did affected you negatively
3. I don't want to be that kind of person, I want to be better than that.
4. This is what I will do in order to be better than that.
Part of 4 is recognizing your father's rights and feelings, just as you wish the same for yourself.
If you can get through all of this discussion and agreement with your father, you will be on your way to the mutual respect, and as much freedom as you and your father can agree on. It might not happen all in one conversation, especially part 4.
This is a good way to apologise in all cases. Saying I'm sorry isn't 1/4th of a meaningful mending of a relationship.
I wish you the fruits of taking responsibility....trust.