Say 1777 Thomas married Maria.
The Albany Lutheran Church lists three baptisms on 22 Aug 1786 with the parents of two of the children being witnesses for the baptisms of two of the other children.
Edward, b. 2 Nov 1785, son of Thomas & Maria Segar, witnesses John & Elisabeth Rewbuttom [Rhubottom]
John, b. 30 Jun 1786, son of John & Elisabeth Rewbuttom, witnesses Joseph Johnson, John Greenwood & Anna May
Josiah, b. 1 Mar 1785, son of John Ashmore & Elisabeth Rothbone, witnesses Thomas & Maria Segar
There was an Alexander Seger, husband of Mary Potter, about the same age as Thomas with children baptized at the the Albany Lutheran Church. It’s been suggested that Alexander was a son of Staats Seger & Susanna Bradt, but that’s unlikely since the names of Alexander’s children don’t correspond. There was an earlier Alexander Seger in Albany who may have been Alexander’s father.
Thomas and Maria Seger were also witnesses the next year for the 11 Mar 1787 baptism of
John, b. 17 Jan 1787, son of Peter & Rebecca Catell [?Peter D. Catell, b. 10 Jul 1760 at Upper Makefield, PA]
The New York genealogical and biographical record volume 35 (1904) has the Amenia, N.Y., Church Records. They’re scattered through the volume starting on page 61 with the baptisms of 1749. The church is “The Old Red Meeting House”. Some of the names have Dutch roots from New York, others English from New England.
Included in it are these baptisms
4 Mar 1779. Thomas Segard and wife had Catharine, b. 29 Mar 1778
11 Jun 1780. Jacob Segar and wife Catherane had John, b. 28 Mar 1780
22 Mar 1783. Thomas Segar and wife Mary had Helen, b. 22 Oct 1781
22 Mar 1783. Thomas Segar and wife Mary had Rosyna, b. 17 Mar 1783