What the Church teaches is absolutely the best way to live one's life. If you fully embrace the Church's view on this, your happiness in this life will be maximized. I really do believe that if you, as an adult, consider every new life to be a gift, however that life comes to be, whatever personal plans you need to change to accommodate that new life...if you consider that life to be just a part of living life to its fullest, you will absolutely maximize your happiness on this earth. If you take a different view, you will fall short of maximizing the possible happiness you could obtain in this world. I believe the Church's teaching is a happiness maximizing teaching.
I am not well equipped to explain the sinfulness of an individual not maximizing his personal happiness on this particular issue. But, I don't have to understand it. I just believe what the Church has taught for 2000 years...and every time I live the way the Church tells me I should live, I end up being a happier person...it really is amazing.
I don't believe conspiracy theories that the Church just wants new followers. If it was all about numbers of butts in the seats, and dollars in the collection basket, they would do a lot of things differently, don't you agree?
Link: http://w2.vatican.va/content/paul-vi/en/encyclicals/documents/hf_p-vi_enc_25071968_humanae-vitae.html