The wobble hypothesis does not discriminate between uracil and thymine. Methylation could favor further stabilization of uracil in the keto form. Thymine is present in keto form only and can pair up but with adenine. Uracil can easily construct the enol form; that is why it forms the U-G pair.
Cite
@article{arxiv.0902.1817,
title = {What the difference between Thymine and Uracil?},
author = {Denis A. Semenov},
journal= {arXiv preprint arXiv:0902.1817},
year = {2009}
}