I beg to differ.
Most people brush this question off with a quick "Jesus died for us, so we can really do whatever we want, as long as we belie-eve, ask for forgiveness and give some more of our money to the church." The bible (and therefore God as well) disagrees.
Hebrews 10:26-29 says: "For if we sin willfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, but a certain fearful looking for of judgment and fiery indignation that shall devour the adversaries. He that despised Moses' law died without mercy under two or three witnesses: Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace?"
I do think that "knowledge of the truth" is a key-element in this text though, and not everyone who says that they are a Christian has come to that state (2Tim.3:7). When you become a Christian, you are born again and you no longer live for yourself. The highest commandment is to love God with all your heart and soul, and to love God is to keep His commandments, and they are not grievous (1John5:3).
To sin deliberately is an act of defiance against God, and "not being perfect" is a very poor excuse. But God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it (1Cor. 10:13). To sin on purpose, knowing that God doesn't want you to do a thing, is indeed spitting on Jesus' cross and counting His blood unholy.