John and Mary would be too young for those marriages if they were baptised as infants, but if they were several years old at baptism, then a possibility.
Thomas 1702 also had a brother John, bapt 27 Dec 1706 at Selmeston, possibly his marriage. There is also a brother Lawrence bapt 27 Dec 1711.
There baptisms at Chalvington (1.5 miles from Selmeston) of Ann 2 July 1697 and Mary 26 Oct 1699, parents Thomas and Mary - might be Thomas jnr's older sisters.
There's a baptism at Rottingdean of a Laurence Newington on 15 April 1740, s/o Thomas and Margaret. Could this be a brother to John and Mary of Semeston and named after Thomas's brother?