The argument that you should not be prepared to receive if you're not prepared to donate is far too simplistic and has far too much of a guilt trip attached to it.
The reality is that you are far, far more likely to need a transplant than to ever be in a position to donate your organs. And let's be honest, the idea of organ donation is generally a hypothetical possibility in the minds of those who sign up to the register, signing up to the donor register doesn't suddenly mean that lives have been saved because you've signed up, it means that if you die suddenly and/or violently in such a way as to make your organs viable for donation then lives might be saved as a result. Whereas if you need a transplant that is a here and now reality which many people may not be prepared for. Do people really think that someone being told they have a life-threatening condition which means they may have weeks/months to live or will have a reduced life capacity unless they receive a transplant should then also be faced with the statement or even question "are you prepared to donate your organs should you die? No? Well then sorry, as you're not on the register you wouldn't be eligible for transplant. We'll put you in touch with the palliative care team then. Oh, prepared to change your mind? Right oh...."?
Where does that end? Never given blood? Oh sorry, best you let this patient bleed to death, can't have them wasting that precious fluid that they wouldn't give back if they could. Didn't donate to a cancer charity? Ah, well why should they be allowed to benefit from the research which donations have paid for.....
As for relatives' consent, well, in truth it's the relatives who have to live with the aftermath of your death not the person who is actually dead. So rather than blame relatives for saying no, surely the issue is down to communication beforehand with regards to whether you would want your organs donated or not?
And with regard to opt-out, I am currently on the donor register, but if opt out ever becomes law in England I will be opting out. Why? Because there is no way that I want to give autonomy over what happens to my body to the state, or the presumption that I am the property of the state unless they decide otherwise. Currently the way opt-out works is that the relatives still have the final say over what happens at the end. It is IMO far more preferable that my next of kin be allowed to make that decision on my behalf than to feel that because I hadn't opted out the state have the ultimate say.
And for anyone who says it won't work like that, are you that confident in our government that future plans won't incorporate such issues? Really? I'm not. My body is mine to decide over. I have no issue with my organs being donated, but that is for me to discuss with the people who will be signing the consent form in the event of my untimely death, not for someone who knows nothing about me to presume on my behalf.