Verify, my understanding of the situation and why your support for gay marriage could prevent you from entering the Church is this:
Even though you don’t agree with the behavior, you support the promotion of a lifestyle that is inherently sinful and destructive both physically and spiritually. This falls under the category of “Scandal.”
From CCC #2284-87 (linked above):
Respect for the souls of others: scandal
2284 Scandal is an attitude or behavior which leads another to do evil. The person who gives scandal becomes his neighbor’s tempter. He damages virtue and integrity; he may even draw his brother into spiritual death. Scandal is a grave offense if by deed or omission another is deliberately led into a grave offense.
2285 Scandal takes on a particular gravity by reason of the authority of those who cause it or the weakness of those who are scandalized. It prompted our Lord to utter this curse: "Whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened round his neck and to be drowned in the depth of the sea."86 Scandal is grave when given by those who by nature or office are obliged to teach and educate others. Jesus reproaches the scribes and Pharisees on this account: he likens them to wolves in sheep’s clothing.87
2286 Scandal can be provoked by laws or institutions, by fashion or opinion.
Therefore, they are guilty of scandal who establish laws or social structures leading to the decline of morals and the corruption of religious practice, or to "social conditions that, intentionally or not, make Christian conduct and obedience to the Commandments difficult and practically impossible."88 This is also true of business leaders who make rules encouraging fraud, teachers who provoke their children to anger,89 or manipulators of public opinion who turn it away from moral values.
2287 Anyone who uses the power at his disposal in such a way that it leads others to do wrong becomes guilty of scandal and responsible for the evil that he has directly or indirectly encouraged. "Temptations to sin are sure to come; but woe to him by whom they come!"90
Because Scandal is sinful and grave matter, it more than likely places you in a state of mortal sin. A baptized person in a state of mortal sin cannot receive any of the Sacraments except Reconciliation and/or Extreme Unction (last rites/anointing of the sick).
Depending on your particular attitude, you may be barring yourself from those Sacraments as well. You do this by not being willing to fully repent of your sins. A confession is made invalid by intentional failure to disclose sin, or by lack of genuine desire to repent. If you have no intention or desire to stop the sinful behavior, you cannot receive the sacramental grace offered. Without this grace, you remain in a state of mortal sin. While in a state of mortal sin, you cannot receive communion.
Another example of this would be divorced and remarried Catholics who have not had their first marriages annulled by the Church. They are barred from the Sacraments because they are living in a state of perpetual sin. They may be able to return to the Sacraments by changing their living situation and/or abstaining from physical intimacy, but a change must be made. Otherwise, every confession they make would be invalid because of their perpetual sin. If they intentionally don’t confess these sins, it is invalid, and if they confess with no desire to change, it is invalid. An invalid confession means they remain in a state of mortal sin. A state of mortal sin means they cannot receive communion (doing so would actually place them in greater peril).