You have to work to maintain your closest friendships when you're in a relationship, that's for sure. But when you're really trying, your friends should see that you're making an effort to keep them in your life, appreciate that effort, reciprocate it, and then be a little understanding when you slip up. Things change when one friend enters a relationship and the other friend is still single.
Some friends may be jealous and upset with you. And while those feelings are normal in this situation, the people who are your real friends, while they might get annoyed with you, will still be there when you come out of "honeymoon" phase. True friends won't accuse you of ditching them for a boyfriend/girlfriend, and then pay you back by turning their backs on you. That's hypocrisy.
That being said, it is good not to go missing or become negligent to others when you enter a relationship. Those other relationships in your life are still very important - where are you going to turn if your relationship doesn't work out but you've let your friendships fall away? Friendships, like any living thing, will die if you don't take care of them. But as long as you make an honest effort not to make everything in your life revolve around your relationship, and you're still good to the people in your life, your friends should understand if you have less free time for them, etc.