Being with your boyfriend's parents may feel like endless hours of pure torture -- or it may feel like just the opposite. Which scenario you experience depends in large part on how you act. Follow the general rules of etiquette while keeping in mind that your boyfriend has been their son for a lot longer than you've been his girlfriend, and you might just find yourself enjoying every minute spent at his home.
Your boyfriend's parents love him, and naturally, they want the best for him. This doesn't mean they should be allowed to judge you based on your actions and appearance, but they will. Dress modestly when you are with his parents, and do not engage in public displays of affection. Hand-holding or a quick peck on the lips is OK, but not extended periods of kissing, and certainly not fondling. Don't sit on his lap or wrap yourself around him -- you may see it as claiming your territory, but his parents will likely see it as embarrassing, impolite and extremely tasteless.
Act just as you would around other adults, or as you would when a guest in someone's home. Use your manners -- say please and thank you, don't monopolize the conversation, answer questions and express an interest in what his parents are talking about or doing. When around his parents, let them lead the conversation. If they ask you personal questions that make you feel comfortable, smile and change the subject. If they persist, gently tell them that you don't feel comfortable talking about it at this time.
It's a good idea to ask your boyfriend ahead of time if there are any conversational topics you should avoid when around your parents. These may include topics about politics or religion. If your boyfriend's parents ask your opinion about these things, be honest, but be brief and don't get drawn into a debate. Express interest in their beliefs and a willingness to learn more about them without compromising yours. You should also ask your boyfriend about family issues that may be sensitive, such as the death of a child. Even something as simple as the fact that your boyfriend's parents prefer guests to take off their shoes when entering the home should be talked about ahead of time so that you can bring socks to wear.
Your boyfriend's parents probably want to get to know you, not someone you are pretending to be. It's important to be yourself. Otherwise, you could be starting a charade that will go on for a long time. If you find something funny, laugh about it. If you don't enjoy bowling, bowl anyway but suggest an alternative activity next time. Unless it is going to hurt someone or cause you to be rude, honesty is always the best policy -- especially when you are being honest about who you are.